MANAGING SHIFTWORK AND FATIGUE IN LAW ENFORCEMENT AND FORENSIC

LABORATORIES

Scott Shappell, Ph.D., R. Jordan Hinson, B.S., Ali Rasheed, B.S. 

Clemson University 

1 

This report was developed through NIST Cooperative Agreement #60NANB11D035N and completed in January 2013. 
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INTRODUCTION

In the interest of ensuring the accurate collection and 

processing of forensic data the National Institute of 

Standards and Technology (NIST) has commissioned 

Clemson University’s Department of Industrial 

Engineering to review scientific literature which can be of 

practical use for understanding ways to manage fatigue and 

shiftwork in law enforcement and to make 

recommendations for rest and duty restrictions. Similar 

regulations aimed at controlling for fatigue and shiftwork 

have become standards in many prominent fields namely 

healthcare, commercial transportation (e.g. commercial 

truck driving) and aviation. It is important to understand the 

effects of fatigue and shiftwork on law enforcement 

personnel because of the sensitive nature of their work. A 

slipup in the lab or at the crime scene could leave an 

innocent person behind bars, or allow a guilty perpetrator to 

go free. It is well know that errors can happen to the best 

intentioned investigator and that many of the effects of 

shiftwork and fatigue can exacerbate the likelihood that an 

error is committed. 

Due to the nature of Crime Scene Investigation, law 

enforcement officers may be forced to work late at night 

during known circadian troughs. When and where crime 

scenes occur often leaves investigators to collect and 

process data while suffering from the effects of fatigue. The 

hope of this paper is that allowing Forensic Scientists to 

understand the effects of fatigue and shiftwork in their field 

could help them mitigate the negative effects of these 

challenges.

Unfortunately, there is little direct data available to 

identify the ideal work/rest schedule for those involved in 

Forensic Science. For this reason, it was necessary to 

examine the scientific body of work conducted on the 

effects of shiftwork in other industrial settings. Specific 

questions this review will address include:

• Which type of shift rotation is optimal?

• Can individuals adapt to night shifts?

• Are there procedures or processes to improve 

adaptation?

• Do traditional second and third shifts require 

different duty and rest periods? 

• Can a break in duty during a shift extend the duty 

period?

• How much duty time can be extended with follow 

on and/or ‘make-up’ rest? 

• How much ‘make-up’ rest is necessary for an 

extended duty period?

• How is duty and rest time tracked among different 

industries? 

SLEEP

The literature is replete with studies demonstrating the 

impact a lack of sleep, especially quality sleep, has on 

performance. These effects are more prominent and have

greater repercussions in service areas like law 

enforcement, which require due diligence owing to the 

sensitive nature of work. In general, the consequences of 

sleep loss and associated fatigue have been shown to 

negatively affect learning capacity (Curcio, Ferrara et al. 

2006), memory consolidation (Karni, Tanne et al. 1994), 

mood (Dinges, Pack et al. 1997), alertness (Bonnet 1985), 

cognition (Cheshire, Engleman et al. 1992), coordination 

(Nakano, Araki et al. 2001), stress levels (Costa 1996), and 

reaction time (Vgontzas, Pejovic et al. 2007). As a result, 

getting enough sleep can be seen as a safety issue in law 

enforcement (Pedersen 2001). Additionally, these effects 

appear to be independent of the type of work being 

performed as deficits have been documented in areas as 

diverse as education (Curcio, Ferrara et al. 2006), 

healthcare (Montgomery 2007), driving (Lyznicki, Doege 

et al. 1998), manufacturing (Blachowicz and Letizia 2006), 

aviation (Bourgeois-Bougrine, Carbon et al. 2003), and 

more germane to this review - forensic science(Lindsey 

2007). 

Arguably, all humans have experienced mental fatigue 

more than once in their lifetime and many have learned to 

cope with the adverse effects. Even so, fatigue continues to 

be a common causal factor identified in many accidents 

including major disasters like the grounding of the Exxon 

Valdez, the meltdown at the Three Mile Island Nuclear 

Power Facility, and the explosion at the Bhopal Refinery 

(Rosekind, Gander et al. 1994). Nevertheless, while coping 

mechanisms can be incorporated to mitigate the adverse 

impact of fatigue, the only lasting solution is quality sleep 

(Caldwell 1997) and the best way to promote quality sleep 

is through appropriate scheduling and rest requirements 

(Haus and Smolensky 2006).

Issues associated with how much sleep individuals 

need, when that sleep should be obtained and what quality 

sleep entails continues to be debated by scientists, 

management, and the workforce. However, one thing we 

can all agree on is that out of necessity, all humans must 

sleep. However just as eating does not make one a 

nutritionist neither does sleeping make everyone an expert 

in sleep and fatigue. Therefore, to fully understand human 

sleep one must first understand a little about circadian 

rhythms and sleep architecture.

Circadian Rhythms

The word “circadian” is actually derived from two 

Latin words circa (about) and dias (day). Indeed, circadian 

rhythms are biological processes that in most environments 

cycle about a normal 24-hour day regulating a variety of 

physiological processes, including: body temperature, 

neurotransmitters, hormones like cortisol and melatonin, 

heart rate, and core body temperature (Blatter and Cajochen 

2007) . Indeed, the latter, core body temperature, may be 

the most sensitive and easily obtained measure of circadian

rhythmicity. However, as important as those physiological 

processes are, even more important to forensic science is 

that performance is also directly related to one’s circadian 

rhythm and the sleep/wake cycle (Fuller, Gooley et al. 

2006). 

As can be seen in Figure 1 the circadian rhythm and as 

a result, performance and sleepiness/alertness, generally 

reach a minimum between 3 a.m. and 6 a.m., known as the 
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circadian trough (nadir), and a maximum (peak) between 2 

and 8 p.m. (Waterhouse, Drust et al. 2005). 

Figure 1. The human circadian rhythm. 

Thought to be generated by the suprachiasmatic 

nucleus within the hypothalamus of the brain, the circadian 

rhythm is actually about 25 hours, not 24 as observed in 

most individuals. However, the circadian rhythm is “reset” 

to 24 hours daily by a series of external references, called 

zeitgebers (Revell and Eastman 2005). The most powerful 

of these zeitgebers is natural daylight (Lack and Wright 

2007). In simple terms, the presence of sunlight signals the 

circadian pacemaker that it is day time, while the absence 

of sunlight indicates night time (Skene and Arendt 2006). 

With a few notable exceptions like Alaska and other polar 

regions where the normal light-dark cycle can be markedly 

different, daytime and nighttime cycles about a 24-hr day.

While sunlight is arguably the most important zeitgeber 

other references like social cues, meal times, exercise, and 

levels of naturally occurring hormones produced by the 

body can influence circadian rhythmicity (Eastman, Hoese 

et al. 1995; Revell and Eastman 2005; Grandin, Alloy et al. 

2006). Regardless of the mechanism however, it is 

important to note that, the circadian pacemaker remains 

remarkably stable from day-to-day even in the presence of 

abrupt external changes such as sleep deprivation and/or 

time-zone changes (Beersma and Gordijn 2007).

Although consistent within an individual, circadian 

rhythms do vary slightly from person to person particularly 

as one gets older (Akerstedt 2007). For instance, there is 

some evidence that a subset of the population are naturally 

suited to day work (so-called morning larks) and others to 

night work (night owls) - yet this may be more subjective 

than physiological. Likewise, as people age it appears that 

their circadian rhythms shifts earlier in the day causing 

earlier rise and bed times (Gander, Nguyen et al. 1993).

Stages of Sleep

Recall that just as performance cycles about the 

circadian rhythm so do sleepiness and fatigue. Fortunately, 

sleepiness and fatigue are acute conditions typically 

relieved by a good night’s sleep. Nevertheless, sleep is not 

just an opportunity to recharge one’s batteries and dream. 

Nor is all sleep the same. 

Figure 2. The typical sleep cycle for an adult.

Throughout the course of a sleep period, the body shifts 

through several stages of sleep characterized by changes in 

brain activity (Figure 2). The first stage (Figure 2; nonrapid eye movement stage 1 or NREM-1) is brief and 

consists of very light, transitional sleep. The second stage 

(Figure 2; NREM-2) is also a fairly light sleep and typically 

makes up the largest amount of total sleep time over the 

course of a sleep period. The third (Figure 1; NREM-3) and 

fourth stages (Figure 1; NREM-4) represent a deeper sleep 

characterized by slow wave brain activity. It is during these 

deeper slow wave sleep (SWS) stages where the 

recuperative work of sleep takes place. The final sleep 

stage involves rapid eye movement (Figure 2; REM) sleep 

involving brain activity that resembles being awake and is 

often associated with dreaming. While it is still unclear 

what role REM sleep plays, some believe that during REM 

sleep memories are consolidated and learning is reinforced 

(Karni, Tanne et al. 1994). Of note, as people age the 

normal sleep architecture becomes distorted with less time 

spent in SWS and REM sleep and more time in lighter 

stages of sleep (Gander, Nguyen et al. 1993; Van Cauter, 

Leproult et al. 2000). 

The Best Time to Sleep

It is well established that the circadian rhythm plays a 

key role in controlling both the duration and quality of 

sleep (Beersma and Gordijn 2007). As a person nears the 

circadian trough several physiological and psychological 

changes take place including a drop in core body 

temperature, an increase in melatonin levels, and a natural 

increase in the pressure to sleep (Murphy and Campbell 

1997). During the circadian trough a person’s alertness 

level decreases, their cognitive abilities drop, and 

performance reaches a minimum (Nakano, Araki et al. 

2001). Clearly then, it appears that humans are “designed” 

to sleep during the night and operate during the day – we 

are not nocturnal animals. In fact, sleeping outside of the 

circadian trough often results in reductions in total sleep 

time, fragmented sleep, less SWS and REM sleep, and 

performance deficits (Akerstedt, Hume et al. 1997). 

The Effect of Sleep Disruptions

In addition to sleeping during the circadian trough, it 

has been shown that sleep continuity is also a prominent 

factor influencing sleep quality (Wesensten, Balkin et al. 

1999). Evidence suggests that periods of continuous sleep 

allow the body to reach the deeper, more restorative stages 

of sleep. In fact, it has been suggested that sleep which is 

disrupted can actually be more detrimental than no sleep at 

all because the body is unable to reach the deeper more 

restorative stages of sleep (Bonnet 1985).
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Not only does waking up during the circadian trough 

result in a decrease in total sleep time and subjective sleep 

quality but it has also been shown to increase stress levels 

(Kecklund, Akerstedt et al. 1997). To make matters worse, 

when sleep disruptions are combined with the stress 

associated with highly demanding jobs and personal 

troubles, the effects on the individual can be exacerbated. 

In effect, a veritable “Catch-22” is created as disruptions in 

sleep cause stress which in turn can lead to known health 

effects made worse by job and life stress which in turn 

produces further sleep disruptions and other sleep-related 

problems (Akerstedt, Knutsson et al. 2002; Knudsen, 

Ducharme et al. 2007). One need look no further than the 

aviation accident record to find numerous examples of how 

sleep disruptions among aircrew during layovers and while 

off duty have impacted performance

Sleep Requirements

While there are some individual differences, the 

scientists seem to agree that the average person needs 

roughly 8 hours of sleep each night to be properly refreshed 

(Akerstedt, Hume et al. 1997). A decrease by just a few 

hours from the recommended amount to as little as five 

hours of sleep per night can cause serious performance 

decrements (Dinges, Pack et al. 1997). More important, 

successive nights of sleep loss can lead to a condition 

known as sleep debt. 

As the name implies, this sleep debt must be made up 

for to have a full recovery (Hardaway and Gregory 2005). 

Even a loss of 1-hour of core sleep per night can lead to 

performance deficits over time as seen in Figure 3. 

However, consecutive nights of 2, 3, and 4 hours of sleep 

per night can lead to alarming performance deficits. While 

there is not a one-to-one payback of sleep debt, it is known 

that after a week of sleeping five hours a night, it can take 

up to two full nights of sleep to fully recover (Dinges, Pack 

et al. 1997).

Figure 3. Performance deficits as a result of days of 

cumulative sleep loss (Source: Department of Behavioral 

Biology, Walter Reed Army Institute of Research.

Summary

• While all sleep is beneficial, SWS is more 

recuperative.

• The best time to sleep is during the circadian trough 

(between 2200-0800).

• Interruptions of nighttime core sleep reduce sleep 

quality and effectiveness.

• Individuals need roughly 8 hours of sleep to be 

properly refreshed.

• Sleep loss is cumulative and may take multiple 

nights of sleep to fully recover.

SHIFTWORK

Shiftwork has become quite common in the modern 24/7 

society and is typically defined as effort that occurs outside 

the hours of 7:00 a.m. to 6:00 p.m. (Blachowicz & Letizia, 

2006). Estimates put the number of people in the U.S. 

engaged in shiftwork somewhere between 20% and 25% of 

the working population (Akerstedt, 1988; Costa, 1996). 

Law enforcement professionals in particular are at risk of 

mental and physical fatigue due to shiftwork. Forensic 

experts are subject to voluntary and mandatory overtime 

assignments as well as long hours waiting to testify in court

(Lindsey 2007). 

While actual shift schedules vary among industries and 

locations, a typical shift schedule might take the following 

form:

First shift: 7 AM - 3 PM

Second shift: 3 PM -11 PM

Third shift: 11 PM -7 AM

What makes schedules like this a concern within all 

industry, not just forensic science, is that there is clear 

evidence that shiftwork can have a negative impact on 

worker performance and health (Haus & Smolensky, 2006). 

For instance, it is well known that shiftwork is inherently 

fatiguing (Tvaryanas & Thompson, 2006) and that 

prolonged shiftwork can lead to sleep disorders, which in 

turn can cause medical (e.g., gastrointestinal and 

cardiovascular), social, economic, and quality of life issues 

(Schwartz & Roth, 2006). Even rotating shiftwork, as is 

often employed in many industrial settings, can result in a 

variety of health problems affecting body mass index, 

waist-hip ratio, and blood pressure (Sookoian, et al., 2007). 

Shiftwork and the Circadian Cycle

At the root of many of the problems associated with 

shiftwork is the impact on normal circadian rhythmicity. 

Circadian rhythms are biological processes that in most 

environments cycle about a normal 24-hour day and 

regulate a variety of physiological processes, including: 

body temperature, neurotransmitters, hormones (e.g., 

cortisol and melatonin), heart rate, and core body 

temperature (Blatter & Cajochen, 2007). However, as 

important as those physiological processes are, even more 

important to space transportation is that performance is also 

directly related to circadian rhythmicity and the sleep/wake 

cycle (Fuller, Gooley, & Saper, 2006). 

As can be seen in Figure 1 the circadian rhythm, and 

consequently, performance and sleepiness, typically reach a 

minimum (nadir) between 3:00 a.m. and 6:00 a.m., also 

known as the circadian trough, and a maximum (peak) 
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between 2:00 p.m. and 8:00 p.m. (Waterhouse, et al., 

2005). However, unlike the depiction in Figure 1, when left 

to its own devices the internal circadian pacemaker is tied 

to a 25-hour clock, not 24 hours as is observed in most 

individuals. Nevertheless, the circadian rhythm is “reset” 

daily to 24 hours by a series of external references, called 

zeitgebers (Revell & Eastman, 2005) - the most powerful of 

which is natural daylight (Lack & Wright, 2007). 

Figure 1. The human circadian rhythm.

In simple terms, the presence of sunlight signals the 

circadian pacemaker that it is day time, while the absence 

of sunlight indicates night time (Skene & Arendt, 2006). 

With a few notable exceptions like Alaska and other polar 

regions where the normal light-dark cycle can be markedly 

different, daytime and nighttime cycles revolve around a 

24-hr day. While sunlight is arguably the most important 

zeitgeber, other references like social cues, meal times, 

exercise, and levels of naturally occurring hormones 

produced by the body can influence circadian rhythmicity 

(Eastman, Hoese, Youngstedt, & Liu, 1995; Revell & 

Eastman, 2005; Grandin, Alloy, & Abramso, 2006). 

Although capable of flexibility as one transits multiple 

time zones or engages in activity outside the normal 

work/rest cycle, it is important to note that, the circadian 

pacemaker remains remarkably stable from day-to-day 

(Beersma & Gordijn, 2007). However, when normal 

circadian rhythmicity is disrupted by external stimuli like 

the length and intensity of sunlight, social cues, and workrest schedules, circadian desynchronosis can occur 

resulting in observable physiological and performance 

deficits.

In addition to entraining human physiology and 

performance to local light/dark cycles, zeitgebers also 

entrain one’s sleep/wake patterns (Revell & Eastman, 

2005). Indeed, the relationship between circadian 

rhythmicity and sleep is widely accepted as the circadian 

rhythm affects both sleep quality and sleep duration 

(Beersma & Gordijn, 2007). Studies have shown that in 

order to receive the maximum benefits of sleep, one must 

sleep continuously during the circadian trough. It is during 

that time when optimal amounts of deeper, more restorative 

sleep can be obtained (Akerstedt, Hume, Minors, & 

Waterhouse, 1997). 

If sleep is not obtained during optimal times (i.e., 

circadian trough) the restorative nature of sleep may be 

inhibited which when repeated over consecutive days can 

lead to sleep debt. In turn, sleep debt can manifest as 

fatigue, sleepiness, and performance deficits (Hardaway & 

Gregory, 2005).

The inherent problem with most shiftwork is that it 

requires individuals to function during times that they 

would normally be sleeping and to sleep during their 

natural periods of wakefulness. What’s more, many 

organizations require employees to work a given shift for 

several days or weeks, which can have serious 

ramifications in high-risk industries if those shifts include 

all or part of an individual’s circadian trough.

Adverse Impact of Shiftwork

The physiological and psychological effects of shiftwork 

on employees can be significant and long lasting. Sleep 

disturbances, gastrointestinal problems, cardiovascular 

troubles, decreased operator performance, chronic stress, 

and mood disorders are all common maladies experienced 

by shiftworkers. In fact, at least one study has reported that 

reduced sleep opportunities associated with working the 

third shift may reduce life expectancy (Kripke, Simons, 

Garfinkle, & Hammond, 1979). Knowing that, it should 

come as no surprise that roughly 20% of all employees 

engaged in shiftwork leave the job due to problems 

associated with working shifts (Costa, 1996). It is important 

therefore, to understand the underlying causes of these 

problems, the way they develop, and negative effects 

commonly associated with them.

Sleep disturbances

Most studies agree that shiftwork leads to increased 

fatigue, sleepiness, and sleep disturbances among workers 

(e.g., Moore-Ede & Richardson, 1985). Referred to 

clinically as ShiftWork Sleep Disorder (SWSD), it appears 

to be more common among those working rotating shifts 

(e.g., rotating weekly between day, evening, and night 

shifts), followed by fixed shifts and straight daytime 

workers (Schwartz & Roth, 2006). 

SWSD is characterized by two primary symptoms: 1) 

excessive sleepiness during the night work period, and 2) 

insomnia during the daytime sleep period. Both are directly 

related to the body’s inability to properly adapt to the 

shiftwork schedule and are exacerbated by the lack of 

exercise, use of alcohol, and emotion-focused behaviors 

(Samaha, Lal, Samaha, & Wyndham, 2007). Moreover, the 

effects of shiftwork on sleep quality are seen more strongly 

in younger workers (Marquie, Foret, & Queinnec, 1999).

Gastrointestinal system

One of the most commonly reported problems associated 

with shiftwork are gastrointestinal problems (e.g., 

constipation, heartburn, gas, and appetite disruption). In a 

recent study conducted by Caruso, Lusk and Gillespie 

(2004), some 20-75% of shiftworkers reported 

gastrointestinal problems compared with only 10-25% of 

daytime workers. In fact, a meta-analysis of 36 studies, 

including a total of 98,000 workers, showed that night work 

specifically led to gastrointestinal problems at a rate 2 to 5 

times higher than among workers who did not work during 

the night (Costa, 1996). 
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Some have suggested that the problems stem from a lack 

of access to the same nutritional foods available to daytime 

workers. Supporting this assertion was the finding that 

those on a rotating shift schedule have a higher body mass 

index and waist-hip ratio (Sookoian, et al., 2007). 

However, others suggest that inconsistent eating habits, 

circadian desynchronization, and sleep deficits cause these 

problems (Vener Szabo, & Moore, 1989). Regardless of 

their exact cause, the concern among workers and 

employers alike is that these problems can develop into 

chronic disorders like chronic gastritis and peptic ulcers, 

which can have serious consequences (Smith, et al. 2003).

Cardiovascular system

Shiftworkers have also been shown to be at an increased 

risk for cardiovascular disorders, especially myocardial 

infarction. One study in particular showed that shiftworkers 

have a 40% higher risk of severe cardiovascular disorders 

(Boggild & Knutsson, 1999). Although the direct cause of 

this increased risk is not known, lifestyles like smoking, 

poor dietary habits, and chronic fatigue associated with 

shiftworking populations are thought to contribute (MooreEde & Richardson, 1985). Certainly, chronic stress, which 

is often associated with shiftwork, has been shown to 

increase blood pressure, which, in turn, may lead to 

cardiovascular problems (Costa, 1996).

Mood

Given that sleep loss is commonly associated with 

decreased reasoning ability and mood changes, it is not 

surprising that shiftworkers report these problems as well 

(Blagrove & Akehurst, 2001). A comprehensive study of 

23 nurses during their first shiftwork experiences showed a 

significant relationship between circadian rhythm and 

mood. Five particular mood aspects were strongly 

correlated with variations in circadian rhythm: vigoractivity, fatigue-inertia, confusion-bewilderment, 

friendliness, and total-mood-disturbance. Of note, the 

effects were found to be equally severe among workers on 

a 4-on 3-off schedule as on workers on an 8-on 7-off 

schedule. This suggests that mood was fully affected after 

only 4 days on the night shift (Florida-James, 

Wallymahmed, & Reilly, 1996). 

It has been suggested that these mood effects are caused 

by both sleep loss and by the difficultly of maintaining 

familial and social relationships (Costa, 1996; Lac & 

Chamoux, 2004). Furthermore, shiftwork has been shown 

to cause some specific problems among women, in 

particular (Costa, 1996). Indeed, it has been suggested that 

the disruption in the body’s natural rhythms common in 

shiftwork adversely affects the reproductive system in 

many ways including: affecting the regularity of a woman’s 

menstrual cycle, increasing the risk of spontaneous 

abortions, decreasing the rates of pregnancies, and causing 

premature delivery and lower birth weights (Hatch, et al., 

1999; Nurminen, 1989). Furthermore, women with children 

are more likely to suffer increased fatigue, from lack of 

daytime sleep, and increased stress from coping with 

childcare responsibilities (Dekker & Tepas, 1990).

PERFORMANCE 

There are a long list of decrements in performance 

stemming from fatigue and sleep deprivation. Researchers 

have found that these impacts include cognitive 

performance decrements, decreased vigilance and sustained 

attention, performance variability, slowed physical and 

mental reaction time, an increase in the number of work 

related errors, an increased tendency to persistently repeat 

behaviors, an increase in false responding, an increase in 

memory errors, and reduced motivation (Rosekind, et al., 

1995; Rosekind, et al., 1996; Dinges & Kribbs, 1991). 

Lamond and Dawson (1999) have equated the result of 

sleep deprivation to alcohol intoxication. They found that 

sleep deprivation of 20 to 25 hours, dependent on task type, 

is equivalent to performance when blood alcohol is .10%, 

well above the national legal limit of .08%. At this level, 

alertness, judgment, coordination, concentration, reflexes, 

reasoning, and depth perception are all negatively 

impacted. Other performance effects can be determined 

based on time of day, and length of time the shiftworker 

has been on shift type. 

Performance decrements are affected by time of day and 

relation to the circadian rhythm, which result in certain 

tasks being performed better at specific times during the 

day. Scott (1994) states that performance can be generally 

predictable if the type of task is known. Tasks that respond 

to the circadian rhythm are those involving vigilance, 

manual dexterity, boring repetitive tasks, or those requiring 

fast reaction time. If these tasks are performed when body 

temperature is low, performance will be less efficient, and 

will contain more errors. Other tasks such as those with 

high-cognitive requirements may be performed best either 

during the night shift or in the circadian nadir. Tasks 

primarily involved in immediate memory have been shown 

to be performed best in the morning (Folkard & Monk, 

1980), while working memory tasks are performed better 

near the middle of the day (Folkard, 1975). An additional 

consideration is cognitive load. As the memory load 

imposed by the task increases, the performance was 

performed better earlier in the day. 

Figure 2. Adjustment rhythms in different performance 

measures (figure adapted from Monk & Folkard, 1979) BODY TEMPERATURE MANUAL DEXTERITY VISUAL SEARCH ADDITION VERBAL REASONING

0

10

20

30

40

50

60

70

80

90

100

THE VARIANCE ACCOUNTED FOR BY PRE SHIFT SHAPE
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The rate at which task performance responds to shiftwork is

also dependent on the type of task. For example, certain 

tasks respond much quicker than others to changes in 

shiftwork. More complex cognitive tasks were found to 

have a greater phase adjustment than simple repetitive tasks 

and body temperature (Scott, 1994). Folkard & Monk 

(1979) found that mathematical addition and verbal 

reasoning were the quickest tasks to adjust followed by 

visual search, and manual dexterity (Figure 2). The 

memory load of a task may also influence the rate of 

adaptation (Scott, 1994). The performance adaptation 

length should be considered when the pace of rotation 

schedule is determined. 

Rest Requirements Associated with Shiftwork

In general, rest can be defined as the absence of 

movement of an object in relation to its surroundings. 

While this definition sounds plausible when describing cars 

and baseballs, in the context of human performance, the 

idea of rest is somewhat ambiguous. There are a number of 

different kinds of rest, including: physiological rest, 

sensory rest, emotional rest, and mental rest. The type of 

rest most pertinent to this review involves bodily 

recuperation such that human performance is maximized. 

The truth is that quality rest has significant effects on 

operator performance, but few agencies, if any, attempt to 

quantifiably define rest as it relates to the effects on 

operator performance. For example the Federal Aviation 

Administration which is tasked with regulating commercial 

aviation, loosely defines a rest period for aircrew as:

“... a period of time required ... that is free of all 

responsibility for work or duty prior to the 

commencement of, or following completion of, a 

duty period, and during which the ... crewmember 

... cannot be required to receive contact from the 

program manager. A rest period does not include 

any time during which the program manager 

imposes on a ... crewmember ... any duty or 

restraint, including any actual work or present 

responsibility for work should the occasion arise.”

14 CFR §91.1057.

While the definition of rest can be defined in many ways

for most industries, including law enforcement, there is still 

a high value placed on its implementation around the work 

schedule. Table 1 provides a summary of operator rest 

requirements in different industries.

Unfortunately, many of the current regulations fail to 

integrate work-rest requirements into the demands of the 

human circadian rhythm, or the day-night cycle. That is, for 

truck drivers, the 7:00 AM to 6:00 PM (11 hr.) shift is 

numerically (time) equivalent to the 11:00 PM to 10 AM 

(11 hr.) shift, even though the latter directly falls over the 

circadian trough when performance and alertness are 

adversely impacted by fatigue and sleepiness. Napping is a 

common form of rest and has proven highly effective in 

improving operator performance. The next section 

discusses this type of rest in more detail. 

Table 1. Comparison of Rest Requirements for Different 

Industries

Trucking1

Maximum consecutive duty 

time allowed

11 hrs

Minimum rest between duty 

time

10 hrs

Cumulative duty limit after 

work span

60 hrs for 7 days or 70 hrs 

for 8 days

Medical Resident2

Maximum consecutive duty 

time allowed

24 hrs (+6 hrs for auxiliary 

activities)

Minimum rest between duty 

time

10 hrs

Cumulative duty limit (A) 

after work span (B)

A=320 hrs; B=28 days

Commercial Pilot3

Maximum consecutive duty 

time allowed

14 hrs4

Minimum rest between duty 

time

10 hrs

Cumulative duty limit (A) 

after work span (B)

A=500 hrs; B=3 months

1 Federal Motor Carrier Safety Administration, 2005 (49 

CFR §395.5)

2 Accreditation Council for Graduate Medical Education, 

2007

3 Federal Aviation Administration, 2007 (14 CFR 

§91.1057)

4 Maximum flight time for a single pilot is 8 hrs, maximum 

flight time for 2 pilots is 10 hrs. Also extended duty and 

flight times apply to heavy crews of 3 and 4 pilots. 

Impact of Shiftwork Across Industries

The adverse impacts of shiftwork on those working in 

law enforcement include risk of officer injury, the 

possibility of significant civil liability damage awards for 

avoidable accidents, and compromised public safety among 

others (Senjo 2011). While it is unclear within the scientific 

literature what impact shiftwork has on law enforcement, 

its impact on other industries has been well documented 

(Akerstedt, 1988; Costa, 1996; Rosekind, et al., 1994) –

particularly as it relates to accident causation. In fact, the 

accident rate associated with rotating shiftworkers has been 

shown to be twice that of fixed day or night shift 

employees (Gold, et al., 1992). Perhaps we should not be 

surprised that several high profile disasters including the 

Chernobyl and Three Mile Island nuclear reactor incidents, 

the grounding of the Exxon Valdez, and the Bhopal

pesticide disaster, have occurred during the circadian 

trough (Mitler et al. 1988; Dinges, 1995). The good news is 

that many organizations have endeavored to reduce the 
8

increased risk of accidents associated with shiftwork –

some more successfully than others. 

Healthcare

In a landmark report by the Institute of Medicine (IOM), 

it was revealed that as many as 98,000 deaths each year are 

the result of medical error (IOM, 2000). Exactly how many 

of these deaths can be attributed to shiftwork, sleep 

disruptions, and fatigue, is difficult to assess. However, as 

one might expect in any 24/7 operation, roughly one in 

three full-time nurses and other healthcare professionals 

engage in some form of shiftwork or rotating schedule of 

day and night shifts (Blachowicz & Letizia, 2006). In 

addition to normal shiftwork, many others (e.g., medical 

residents, interns, and some practicing physicians) work 

extraordinarily long hours where 24-, 48-, or even 72-hour 

duty is not unusual. As such, they are at a higher risk of 

making serious medical errors (Surani, et al., 2007). The 

issue became so widespread that the Accreditation Council 

on Graduate Medical Education and the Association of 

American Medical Colleges has placed an upper limit on 

how long medical residents can work (i.e., 80 hours per 

week averaged over a two-week period). 

While a reduction from a traditional 100-hour workweek 

to an 80-hour workweek is an improvement, one has to 

wonder if a larger reduction would be even more 

beneficial; particularly, given a recent study that found a 

reduction in hours to a more manageable 63-hour week 

resulted in a sizeable decline in the number of errors made 

by interns (Landrigan, et al., 2004). What’s more, medical 

residents working a restricted schedule that limits

consecutive work hours to 16 hours, have exhibited fewer 

than half as many attention failures as residents on a 

schedule of 24-hours on-duty or longer (Lockley, et al., 

2004). 

Trucking and Highway

Like healthcare professionals, truck drivers also 

frequently work through the night. However, unlike the 

frequent decision-making required of nurses and 

physicians, driving a truck is a skill-based task that requires 

constant vigilance and alertness. Unfortunately, studies of 

truck drivers have revealed that they too experience higher 

levels of sleepiness during the circadian trough. In fact, in 

at least one study, the degree of driver sleepiness could be 

predicted based on total number of work (driving) hours 

and the time of arrival (Kecklund & Akerstedt, 1993). 

However, the threat to safety does not end at the doors of 

the hospital or gates of a factory. Many workers regularly 

commute to and from work in their vehicles, exposing them 

to risks associated with shiftwork and fatigue. In fact, the 

National Highway Traffic Safety Administration (NHTSA) 

estimates that there are some 56,000 sleep related road 

crashes annually, resulting in 40,000 injuries and 1,550 

fatalities. This data suggest that approximately 4% of fatal 

accidents can be attributed, at least in part, to fatigue. 

However, other studies in the U.S. show much higher rates, 

from 17% of accidents overall to 50% of fatal accidents 

(Royal Society for the Prevention of Accidents, 2001). 

While fatigue-related accidents can occur at any time of 

day, a meta-analysis of fatal car accidents conducted by 

Mitler et al. (1988) revealed that considerably more 

accidents occur between midnight and 6:00 a.m. then any 

other time of day (Figure 4). Similar trends have been seen 

within the road transport industry with a peak in accidents 

from 2:00 – 3:00 a.m. and a second minor peak in the early 

afternoon (Folkard, 1997).

Figure 4. Fatigue related fatal car accidents from Israel, 

Texas, and New York (figure adapted from Mitler, et al., 

1988). 

These data are particularly interesting in light of the 

findings of Garder et al (1994) who surveyed over 200 

drivers and found that 31% admitted to falling asleep at the 

wheel on at least one occasion during the past year. 

Moreover, employees who worked extended work hours 

were much more likely to have problems. Specifically, 

shiftworkers who worked extended hours were more than 

twice as likely to be involved in a motor crash and nearly 6 

times more likely to be involved in a near miss than 

employees who worked a standard 8 hr shift (Barger et al, 

2005). In fact, as the number of extended work shifts 

increased per month, the risk of a car accident on the way 

to or from work also increased by over 16%. In months 

where five or more extended shifts were worked, they were 

nearly two and one half times more likely to be involved in 

an accident, and nearly 4 times more likely to fall asleep in 

traffic.

Aviation

Commercial and military pilots suffer from many of the 

same complaints as shiftworkers, including the lack of 

alertness and excessive fatigue. Whether flying multipleleg short-haul domestic flights, or long duration 

international flights, aircrews often operate at all points of 

their circadian clock while transiting multiple time zones 

(Lowden & Akerstedt, 1998). Such operations can lead to 

unintended sleep loss particularly when aircrews are forced 

to sleep during their circadian peak or in unfamiliar 

accommodations away from home.

Unfortunately, the impact of these disruptions can 

threaten the efficient and safe operation of flights. In a 

study of 12 British Airways pilots both subjective and 

objective sleepiness increased during flights through the 

night. More alarming, of the 12 pilots studied, ten either 

Midnight

International 

Data

N=6052

6 AM Noon 6 PM Midnight

100

200

300

400

500

600

700

800

900

1000

1100

1200

Time of Day

FATIGUE-RELATED ACCIDENTS
9

involuntarily slept or exhibited symptoms of sleepiness 

during the flight (Wright & McGown, 2001).

Several scientific surveys have exposed similar 

problems. For instance, a survey of both long- and shorthaul commercial airline pilots revealed that many pilots 

have experienced fatigue effects directly related to 

shiftwork including small errors and slow response rates 

(Bourgeois-Bougrine, et al., 2003). In another study, nearly 

all the U.S. Army aviation personnel surveyed reported that 

they had engaged in some form of shiftwork with nearly 

two-thirds reporting that they did not get enough daytime 

sleep to properly adjust to the shift change (Caldwell & 

Gilreath, 2001).

While different than either military pilots or those 

involved with commercial short-haul domestic flights, 

pilots involved in international flights are often required to 

fly multiple legs outside of normal working hours over a 

period of several days. Given the 19-22 hours that are 

standard wakefulness hours for commercial pilots on 

outbound international flights, it should come as no 

surprise that significant levels of fatigue and performance 

decrements limits the ability of pilots to visually search 

their environment – a particularly troubling finding given 

the visually demanding task of flying an aircraft (Suvanto, 

Harma, Ilmarinen, & Partinen, 1993). These decrements are 

often worse during later flights in a multi-leg trip (Petrilli, 

Roach, Dawson, & Lamond, 2006).

Complicating matters, individuals (both aircrew and 

passengers) who transit multiple time zones during long, 

international flights often find their internal circadian 

rhythms desynchronized from the local light-dark cycle and 

other zeitgebers. This condition, colloquially referred to as 

jetlag, creates many problems and discomforts for the 

modern traveler. Unfortunately, the many bodily functions 

regulated by the circadian cycle adjust to new time zone at 

different rates, sometimes taking more than a week to fully 

adjust and resynchronize (Wegmann & Klein, 1985). It is 

during these times where symptoms of jet lag (e.g., fatigue, 

increased reaction time, reduced attention, diminished 

memory, and withdrawn mood) manifest.

As a result, when a passenger or crewmember arrives, 

Shiftwork presents a similar, but more challenging 

problem, as the external cues are continually fighting 

against the internal circadian cycle. While the worker 

attempts to sleep during the day and work at night, external 

cues such as the natural light/dark cycle and social cues 

continually fight adaptation. The strongest zeitgeber, the 

natural light/dark cycle, in particular is in direct 

competition with shiftwork adaptation (Lack & Wright, 

2007). While this type of exogenous indicator is normally 

very useful in regulating the circadian rhythm, it can make 

adaptation to the night shift quite difficult as the body is 

constantly fighting to return to a typical day/night schedule 

– particularly on days off when individuals often revert to 

normal daytime schedules.

Firefighting

Firefighters face a different challenge since they may 

have the opportunity to sleep during their shift; but, are 

subjected to sleep disruptions and abrupt waking during 

emergencies. A study involving waking firefighters up at 

different times during the night showed higher levels of 

subjective sleep complaints and irregular sleeping patterns 

particularly when awakened during the circadian trough 

(Takeyama, et al., 2005).

COUNTERMEASURES TO FATIGUE AND 

INSOMNIA

Ideally, law enforcement personnel and forensic 

experts would always have the opportunity to obtain the 

quantity and quality of sleep necessary to ensure maximum 

performance because their work is vital to public safety. 

Unfortunately, there are many circumstances in which 

adequate sleep cannot be obtained. In these cases there are 

some options available to minimize the effects of 

inadequate sleep. In the case of fatigue, these are only 

short-term fixes and should not be relied upon as long-term 

solutions. Some commonly used countermeasures to 

fatigue and insomnia are presented in Table 1 and are 

discussed briefly below. 

Table 1. Countermeasures to fatigue and insomnia.

FATIGUE INSOMNIA

Non-Pharmacological Non-pharmacological

Napping Diet

Light Exposure

Physical Activity

Exercise

Pharmacological Routine Sleep Times

Caffeine Pharmacological

Other Stimulants Melatonin

Alcohol

Sedatives

Fatigue

Napping - Many people experience some form of 

fatigue during the following day as a result of sleep 

loss/disruptions. In these instances, daytime napping has 

been found to be an effective non-pharmacological 

countermeasure to fatigue. Napping has also been found to 

be especially beneficial when taken before the night shift or 

when an individual is required to remain awake when 

he/she would otherwise be asleep (Petrie, Powell et al. 

2004). 

That being said, the larger question is “how long does a 

nap have to last to be effective?” Not surprising, naps of 1-

2 hours have been shown to significantly increase the level 

of alertness and to a lesser extent the level of performance 

(Vgontzas, Pejovic et al. 2007) and memory (Tucker, 

Hirota et al. 2006). However, even naps as short as 10 

minutes can be restorative (Brooks and Lack 2006). Still 

others have demonstrated that naps as brief as 15 to 20 

minutes increase the alertness levels of workers and 

increase the accuracy in vigilance tasks (Purnell, Feyer et 

al. 2002; Takahashi, Nakata et al. 2004). 

Curiously, the point in the circadian cycle when a nap 

occurs does not seem to influence accompanying 

improvements in alertness, meaning that the time a nap is 

taken does not affect the benefits of the nap. This lends 
10

itself to the practice of law enforcement departments 

encouraging officers to take naps whenever their workload

is low enough (Pedersen 2001). Furthermore, the effects of 

sleep inertia, grogginess immediately following waking, do 

not appear to be a concern when short naps (i.e., less than 1 

hour) are taken (Driskell and Mullen 2005). 

Light Exposure - Exposure to bright light is another 

effective means to address fatigue. Recall that the 

light/dark cycle in humans plays an important role in 

synchronizing the circadian pacemaker (Duffy, Kronauer et 

al. 1996). Not surprising then, the exposure to specific 

types of bright light (i.e., those that simulate the natural 

wavelengths seen in sunlight) may be used to phase delay 

the body’s circadian rhythm in some operational settings 

(Boivin and James 2002) and may also help in overcoming 

the effects of jet-lag due to rapid scheduling changes 

(Wetterberg 1994; Caldwell 2005; Revell and Eastman 

2005; Lack and Wright 2007). 

Physical Activity – Some agencies encourage officers 

to use physical activity as a means to stay awake. When 

fighting sleep, using a form of transportation, like a bicycle 

or “walking the beat”, as opposed to sitting in a vehicle 

requires physical activity, which helps prevent the 

immediate onset of sleep (Pedersen 2001). In parallel, a 

criminologist could break away from the seated 

workstation by electing a task that requires a greater level 

of physical activity when feeling fatigued.

It has been shown that a single 4-hour exposure to light 

on the first day of the night shift can phase-delay the 

secretion of melatonin by 2-hours and thus delay the onset 

of sleepiness and fatigue and improve performance on the 

night shift (Dawson and Campbell 1991). Dark glasses may 

also help avoid resynchronization to the natural circadian 

rhythm and allow workers to adapt more quickly to 

nighttime shift work (Eastman, Stewart et al. 1994; Yoon, 

Jeong et al. 2002). There is also some evidence that bright 

light exposure during the day may help with fatigue by 

promoting better nocturnal sleep for workers (Wakamura 

and Tokura 2000). 

Caffeine - Because of its abundance, over the counter 

availability, minimal side effects and low cost, caffeine is 

perhaps the most commonly used pharmacological 

countermeasure to fatigue. The use of caffeine improves 

performance and vigilance during periods of sleep 

deprivation (Caldwell 2005; Dagan and Doljansky 2006;

Phillip, Taillard et al. 2006). Caffeine also improves 

reaction time. The use of 300-mg of caffeine may sustain 

performance levels for a period of 24 hours (Caldwell and 

Caldwell 2005). 

In fact, some have suggested that when sleep needs to 

be delayed for a longer period of time, low doses of slow 

release caffeine may be a better countermeasure to fatigue 

than a nap (De Valck, De Groot et al. 2003). Then again, 

the use of caffeine as a countermeasure to fatigue might not 

be effective for all people. For instance, frequent users of 

caffeinated products may build a tolerance that renders the 

benefits of caffeine useless (Caldwell 2005). After 18 days 

of chronic use, many if not all individuals experience a 

complete tolerance to the effects of caffeine (Caldwell and 

Caldwell 2005).

Stimulants – Although less common in most civilian 

settings, the use of prescription and over the counter 

stimulants has also been used as a countermeasure to 

fatigue. For example, amphetamines have been used in 

some military operations. Likewise, dextroamphetamine 

has been shown to improve pilot performance during 

sustained wakefulness (Caldwell 2005; Caldwell and 

Caldwell 2005; Wesensten, Killgore et al. 2005; Eliyahu, 

Berlin et al. 2007) while methamphetamine appears to 

improve performance in shift workers (Hart, Haney et al. 

2005). Regardless, the use of these stimulants is forbidden 

in law enforcement settings. 

Notably, the use of stimulants is accompanied by side 

effects which include the possibility of addiction and 

cardiovascular problems (Caldwell 2005). However, more 

recently, the use of modafinil, a stimulant boasting fewer 

side effects than traditional stimulants, has been researched 

as an alternative to other stimulants (Caldwell 2005; Gill, 

Haerich et al. 2006; Eliyahu, Berlin et al. 2007). Modafinil 

has also been given in multiple doses over a work shift to 

sustain wakefulness and improve vigilance (Lagarde and 

Batejat 1995). 

Insomnia 

Sleep hygiene refers to a group of behaviors that are 

typically utilized along with pharmacological aids to reduce 

insomnia. Like countermeasures addressing fatigue, those 

effective in reducing insomnia generally fall into two 

categories: non-pharmacological and pharmacological 

counter-measures.

Diet - One aspect of good sleep hygiene is diet. While 

the reasons for insomnia in shift workers vary, in general 

differences in the timing of meals, the unavailability of 

well-balanced meals at night, and snacking all contribute to 

difficulties with sleeping (Atkinson and Davenne 2007). 

That being said, people who suffer from insomnia should 

avoid large meals (Morin 2006) and foods with high sugar 

or caffeine contents before bedtime (Atkinson and Davenne 

2007).

Exercise - Another aspect of good sleep hygiene is 

exercise. Exercise later in the day can increase the depth 

and length of sleep during the night (Caldwell 1997). It 

appears that the increase of body temperature from exercise 

is associated with a faster sleep onset (Stepanski and Wyatt 

2003). However, the timing of the exercise is important. 

Evidence suggests that exercise should be completed 5-6 

hours before bedtime and avoided within 3 hours of sleep 

(Morin, Hauri et al. 1999; Morin 2006). 

Routine sleep times - Creating a nighttime routine is 

also part of good sleep hygiene. Having a consistent 

bedtime and wake-up time may help in the treatment of 

insomnia (Caldwell 1997; Morin, Hauri et al. 1999). It has 

been suggested that having a consistent bedtime will begin 

to build a connection between the time of day and sleep, 

and eventually your body will start to automatically fall 

asleep at the same time every night (Caldwell 1997).
11

Melatonin - Melatonin is a naturally occurring 

chemical in the body that is known to induce sleep. As one 

might expect, the release of melatonin in the body depends 

on retinal light exposure. Put simply, sunlight inhibits 

melatonin release while the lack of sunlight permits natural 

melatonin stores to be released, inducing the onset of sleep 

(Cardinali, Furio et al. 2006; Skene and Arendt 2006). 

If administered properly, exogenous melatonin 

(melatonin not naturally made by the body) administration 

has been used to phase shift the sleep cycle to overcome 

shift work or jetlag (Melatonin 2005; Skene and Arendt 

2006). To overcome jetlag and shift work the onset of sleep 

either needs to be sped up or slowed down. When traveling 

eastwards or trying to sleep after working the night shift, 

administration of melatonin will speed up the onset of sleep 

and allow those affected to get onto the sleep cycle they 

desire. Melatonin can also be used to speed up the initial 

onset of sleep for people with insomnia. 

Sedatives - Sedatives have been commonly used as a 

countermeasure to insomnia within the general population. 

However, within the civil aviation field, the use of 

sedatives and other hypnotics which induce sleep are 

generally banned (Caldwell 2005). Nevertheless, sedatives 

have been shown to be useful for inducing sleep at times 

when or in places that are not necessarily conducive to 

sleep (Caldwell and Caldwell 2005). 

When considering the use of sedatives, care must be 

taken to choose the correct one. The available time for 

sleep must be considered, as sedatives with long half-lives 

(i.e., the time it takes to decay to half of the initial value), 

can cause performance decrements if a user is suddenly 

awoken for duty (Caldwell 2005; Caldwell and Caldwell 

2005). 

Antihistamines can also cause a sedative effect. 

However, like sedatives many antihistamines can produce 

drowsiness and decreased performance (Verster and 

Volkerts 2004). Notably however, all sedatives have 

detrimental effects on normal sleep architecture, the overall 

structure of a night’s sleep. Sleep architecture includes 

succession of sleep cycles, the number of stages, and the 

length of time spent in each stage (Hirshowitz 2004). 

Alcohol - The use of alcohol as a countermeasure to 

insomnia is generally not recommended. While the 

consumption of alcohol before bedtime has been shown to 

speed up the onset of sleep, like other sedatives, alcohol is 

known to disrupt normal sleep architecture leading to 

feelings of fatigue and sleepiness upon awakening (Vitiello 

1997).

OPERATIONAL QUESTIONS AND 

RECOMMENDATIONS

The effect of shiftwork on performance and fatigue in 

the law enforcement community and other industries is a 

complex topic that continues to attract the attention of 

scientific and operational communities. While there are 

literally hundreds of references on the topic, the previous 

overview has provided a foundation from which to answer 

the questions regarding shiftwork posed at the beginning of 

this report. Note that where scientific data exists, answers 

to each question and specific recommendations will be 

given. Where limited or no information is available, the 

reader will be advised and recommendations based on best 

human factors principles will be provided.

WHAT TYPE OF SHIFT ROTATION IS OPTIMAL?

Although shifts vary widely depending on operational 

needs, the use of three 8-hour shifts (i.e., morning, 

afternoon, and night shifts) is common among 24/7 

operations including forensic science. Of the three shifts, 

arguably the third (i.e., night) shift garners the most 

attention given that it occurs during the circadian trough – a 

period of time associated with known performance deficits. 

So, what type of shift rotation could minimize the adverse 

impact of operating during the circadian trough? 

Trying to document how organizations manipulate shift 

rotations is difficult. However, using the night shift as a 

benchmark, the way organizations rotate workers among 

shifts can generally fit into one of four categories 

(Wilkinson, 1992):

1) A permanent (fixed) night shift, where employees 

spend all their time working the night shift.

2) A prolonged night shift, where employees work the 

night shift for a period of weeks to months.

3) A weekly rotating night shift, where employees stay 

on each shift for one week before rotating to a 

different shift.

4) A rapidly rotating night shift, where an employee is 

on the night shift for one to three days before rotating 

to a different shift or day(s) off.

In effect, the four types of shift rotation can be reduced 

into two categories: those with permanent or prolonged 

rotation schedules and those that involve some form of 

short-term (i.e., weekly/daily) rotation through the shifts. 

Exactly which type of shift scheduling is optimal (i.e., 

permanent/prolonged schedules or rotating schedules) 

continues to be a matter of debate within the scientific 

literature.

In an extensive review of the scientific literature, 

Wilkinson (1992) advocated the use of 

permanent/prolonged shift schedules based on the 

following findings from a number of studies:

• Employees engaged in permanent/ prolonged night 

shift schedules appear to sleep more than those 

working rapidly rotating shifts.

• When uninterrupted by days off, the circadian cycle 

of those working a permanent night shift can adjust 

almost completely.

• In some cases, performance on the night shift may 

be better for those working permanent/prolonged 

night shifts than rotating shifts.

• Sickness/absenteeism appears to be less prevalent 

for those working permanent and rapidly rotating 

shifts than for shifts that rotate weekly.

Although Wilkinson’s observations may lead one to 

conclude that permanent/prolonged schedules are best, 

other data suggests that rotating schedules may be 

preferred. For instance, if you ask employees they clearly 

favor rotating schedules over permanent/prolonged 
12

schedules (Wilkinson, 1992). In fact, employees often 

prefer rapidly rotating schedules like those used by air 

traffic controllers (i.e., two afternoon shifts followed by 

two day shifts and a night shift in a five day period) 

because it maximizes time off between workweeks (Della 

Rocco & Cruz, 1996). After all, it is very difficult to 

maintain social relationships while working the evening 

and night shifts when nearly everyone else is on a normal 

day/night schedule. 

Some have even suggested that in addition to 

maintaining normal social relationships, rotating shift 

schedules moderate the disruption to circadian rhythms and 

reduce sleep loss (e.g., Knauth, 1993; Knauth, 1996; 

Wedderburn, 1992; Folkard, 1992; Tepas et al., 1985). 

Even supporters of permanent schedules agree that the 

circadian cycle never fully adapts to a peak at night and 

trough during the day since most individuals return to the 

normal sleep/rest schedules of those around them on their 

days off. Consequently, any adaptation of the circadian 

cycle to night shiftwork is frequently disrupted (Wilkinson, 

1992). Some evidence suggests, however, that if permanent 

or prolonged night shifts are utilized it is better to adopt a 

“many” consecutive night (i.e., 5-7 nights) pattern than a 

“few” (i.e., 1-4 nights) (Dirkx, 1993).

If an organization chooses to employ a rapidly rotating 

shift, does it matter which direction the shifts rotate? In 

other words, a shift schedule can rotate forward (i.e., one or 

more morning shifts followed by one or more afternoon 

shifts followed by one or more night shifts), backward 

(night/afternoon/morning), or be a hybrid of the previous 

two. Given that the endogenous circadian clock actually 

runs at 25 hours it would appear that forward rotating 

schedules would be favored since individuals could extend 

their day as the shifts are rotated (Knauth, 1993; Knauth, 

1996; Barton & Folkard, 1993).

Police agencies are encouraged to use fixed shifts as 

much as possible and avoid rotating shifts in particular 

backward rotating shifts (Pedersen 2001). For example, 

many U.S. air traffic controllers choose rapidly rotating 

counter-clockwise schedules over other schedules (Note: 

there is not a single prescribed shift schedule rotation for 

U.S. air traffic controllers). Referred to as a 2-2-1 schedule, 

air traffic controllers begin with two evening shifts (e.g., 

3:00 p.m. to 11:00 p.m.) on successive days followed by 

two morning shifts (e.g., 7:00 a.m. to 3:00 p.m.) 

immediately followed by a night shift (e.g., 11:00 p.m. to 

7:00 a.m.). Note that the night shift often begins the same 

day as the last morning shift leaving little opportunity for 

sleep. While this may maximize time off duty (i.e., 3 days 

of a 7 day work week) for the air traffic controller, several 

studies have shown that rotating schedules often lead to a 

reduction in core sleep, the rapid accumulation of sleep 

debt, and most importantly, performance deficits (Della 

Rocco & Cruz, 1996; Cruz et al., 2003; Signal & Gander, 

2007). These rapidly-rotating backward schedules continue 

to be a concern within the FAA. 

Summary and Recommendations

• There does not appear to be any clear consensus 

regarding the best way for employees to rotate through 

shifts. However, all agree that organizations should 

avoid repeatedly disrupting the circadian cycle over the 

long term.

• For organizations where safety is vital or where the 

adaptation of the circadian cycle is not interrupted by 

other factors (e.g., schedule changes on off days), 

permanent shift schedules should be utilized. However, 

if either situation is not achieved, a rapidly rotating shift 

schedule is the “best compromise” (Folkard, 1992).

• If rotating shifts are employed, they should rotate 

forward (i.e., morning/afternoon/night) rather than 

backward.

• Individuals on rapidly rotating shift schedules appear to 

be happier due to improved social relationships - albeit, 

performance and sleep may be sacrificed. In addition, a 

rapidly rotating shift schedule keeps the alertness high 

and only incorporates a few bad “day-sleeps” (Monk, 

Folkard, & Wedderburn, 1996).

• If a permanent or prolonged shift schedule is utilized, a 

many-night schedule should be followed (Dirkx, 1993).

CAN INDIVIDUALS ADAPT TO NIGHT SHIFTS?

There is little evidence that individual circadian rhythms 

ever fully adapt to the night shift when individuals are 

repeatedly exposed to normal daytime zeitgebers like 

sunlight, meal cues, and social cues. Even if individuals 

access meals and social interactions during the night rather 

than the day, many night shifts end around 7:00 a.m. 

exposing shiftworkers to ambient sunlight on the drive 

home, thereby entraining the endogenous circadian rhythm 

to the local light/dark cycle with a peak during the day and 

a low during the night. As a result, night shiftworkers sleep 

less during the day and suffer from performance deficits 

typical of operating in the circadian trough (Folkard, 1992).

That being said, can shiftworkers adapt to night shift 

schedules at all? The answer is “it depends.” One 

community that employs permanent/prolonged night shifts 

is nursing. Dirkx (1993) examined two groups of night shift 

nurses: 1) those that work the night shift for one to four 

days before having a night(s) off, and those that work the 

night shift for five to eight nights in a row before having 

night(s) off. On the surface, both appeared to adapt to 

permanent night work. However, it was found that those 

that work five to eight nights in a row tend to stay in bed 

longer (presumably sleeping), which, in the long run, may 

be more adaptive.

It is important to note that while individuals tend to 

“adapt” to the night shift, that does not necessarily mean 

that they perform better or that the risk of 

incident/accidents is any less. Indeed, when the a priori

risk is constant from shift to shift, the relative safety risk is 

lowest for the morning shift and is highest for the night 

shift with the afternoon shift being somewhere in between. 

Moreover, during the night shift the relative risk is highest 

during the first hours of the shift and reaches its minimum 

near the end (Folkard & Tucker, 2003). 

Perhaps more important, the relative risk also increases 

over the first four consecutive night shifts (Figure 3; 

Folkard & Tucker, 2003) although the possibility of a 

decline over several consecutive night shifts is possible 
13

(Folkard & Lombardi 2004; Wilkinson, 1992). In other 

words, while we may do well with one night’s sleep loss, 

risk increases as the workweek continues and sleep debt 

accrues.

Curiously, when the a priori risk of injuries remains 

constant throughout the day, fewer injuries were reported 

on the night shift then the morning shift. However, Folkard 

and Tucker (2003) attributed this to the usual lack of a 

nurse on the night shift and thereby fewer reported injuries. 

On the other hand, many morning shifts start at 7:00 a.m., 

which may require employees to wake at 5:00 a.m. or 

earlier to report for duty thereby cutting into core sleep.

Figure 3. Relative Risk over Successive Nights (figure 

adapted from Folkard & Tucker, 2003)

So, while risk appears to increase on successive nights 

working on the night shift, do some people adapt better 

than others? Indeed, it does appear that some individuals 

are better suited to adapt to night shiftwork than others. 

For example, some individuals simply have an ability to 

overcome drowsiness, easily modify their sleeping habits, 

and show lower anxiety manifestation; all of which are 

associated with a higher level of tolerance to shiftwork 

(Costa et al., 1989). Note however, that while these 

characteristics helped individuals “cope” with the adverse 

impact of shiftwork they did not have an effect on the 

endogenous circadian rhythm. Indeed, their circadian 

rhythms remained entrained to the local light/dark cycle. 

It also appears that shiftwork is tolerated better by 

younger than older (i.e., late 40’s and early 50’s) adults. 

This may be due to the fact that as we age our circadian 

rhythm flattens out and moves to earlier times of the day 

(Carrier, Monk, Buysse, & Kupfer, 1997; Brock, 1991). In 

addition, normal sleep architecture becomes distorted as 

less time is spent in SWS and REM sleep and more time is 

spent in less restorative, lighter stages of sleep (Gander, 

Nguyen, Rosekind, & Connell, 1993; Van Cauter, Leproult, 

& Plat, 2000; Carrier, et al., 1997). 

Personality and chronotype (i.e., the preference to be 

active early or late in the day) may also influence an 

individual’s ability to adapt to night shiftwork. Specifically, 

morning-types and introverts both have an earlier phase 

than night owls or extroverts (Baehr, Revelle, & Eastman, 

2000; Duffy, Dijk, Hall, & Czeisler, 1999; Kerkhof, 1985; 

Kerkhof, & van Dongen, 1996). In fact, extreme morning 

types wake up about 80 minutes earlier than evening types, 

which can make the transition to the night shift even more 

difficult (Kerkhof, 1985).

The second component is sleeping itself. In general, 

those who rate themselves as more flexible sleepers (i.e., 

less effected by environmental conditions) and report that 

they need less sleep often fare better in the shiftwork 

environment. After all, those working the night shift are 

forced to sleep when ambient noise and other 

environmental conditions (e.g., light, temperature, etc.) are 

less than optimal. This may help to explain why 

shiftworkers usually report getting about 7 hours less sleep 

per week than their non-shiftworking counterparts (Monk, 

1997). 

The third component involves social and domestic 

factors. A high domestic load or family pressure to revert to 

a more normal work/rest schedule during days off can 

make it impossible for some shiftworkers to maintain a 

nocturnal lifestyle. When facing social or domestic 

pressures, shiftworkers often go back to a more normal 

9:00 a.m. to 5:00 p.m. shift schedule, which makes it much 

harder when their night shifts resume.

Other factors that can be aggravated during shiftwork are 

sleep disorders and health problems, including insomnia, 

gastrointestinal problems, epilepsy, and heart disease. 

Some shiftworkers even resort to chronic alcohol and drug 

use to cope with family problems or to help them sleep. 

The problem is that alcohol and drugs often lead to a 

decrease in sleep quality and quantity, which in turn 

exacerbates any problems associated with shiftwork. In the 

end, a vicious cycle is set in motion, which leads to more 

issues. As a result, many companies require that 

shiftworkers do not have any of these problems prior to 

starting shiftwork.

Summary and Recommendations

• Adaptation to the night shift is more difficult than either 

the morning or afternoon shifts, respectively. 

• In general, while people adapt mentally to shiftwork that 

includes night shifts, performance deficits and the 

relative risk associated with normal circadian variation 

remain.

• Side effects for shiftwork (particularly those employing 

night shifts) include sleep disorders, gastrointestinal 

problems, social problems, cardiovascular problems, 

chronic stress, and performance deficits.

• Employees should be counseled against the use of 

alcohol and other sleep aids (either prescription or nonprescription) since they may delay adaptation and 

exacerbate fatigue associated with shiftwork.

• Where possible organizations should screen employees 

for health problems that can be aggravated by shiftwork.

ARE THERE PROCEDURES OR PROCESSES TO IMPROVE 

ADAPTATION?

By its very nature the circadian cycle is relatively 

inflexible; however, the body does retain the ability to 

slowly adapt to changes within 1.5 hours in either direction 

0.9

1

1.1

1.2

1.3

1.4

1 2 3 4

RELATIVE RISK

SUCCESSIVE NIGHTS

RELATIVE RISK OVER SUCCESSIVE NIGHTS
14

from its natural period of 25 hours (Moore-Ede & 

Richardson, 1985). Unfortunately, when the circadian 

rhythm is forced to shift in excess of 1.5 hours, as would be 

the case with shiftworking populations, serious problems 

can occur. While many shiftworkers simply learn to cope 

and/or adapt to the effects, others never fully do (Thorne et 

al., 2006). 

Ideally, shiftworkers would always have the opportunity 

to obtain the quantity and quality of sleep necessary to 

ensure maximum performance. Of course, this assumes that 

shiftwork does not impede normal sleep times. Clearly, that 

is not the case for individuals working the third shift. 

Depending upon when the morning shift starts, the same 

can be said for those working the morning shift as well. 

While optimal starting and ending times for each shift do 

not exist, management should avoid early start times in the 

morning shift mainly because it requires that employees 

wake up during the lowest part of the circadian trough and 

may impact sleep quality, performance, and safety. On the 

other hand, while a late start (e.g., 9:00 a.m.) may be 

preferred by those working the morning shift, a late start in 

the morning equates to a late end for the afternoon shift and 

a late start for the nightshift, all but ensuring that night shift 

workers will not adapt to a nighttime schedule.

Regardless of when a shift starts and ends, there will 

always be shiftworkers that simply cannot obtain adequate 

sleep. As with anyone who fails to get adequate sleep, there 

are some options available to improve sleep quality during 

the day and enhance the opportunity to adapt to the 

shiftwork quickly. Note however that in most cases these 

are only short-term fixes and should not be relied upon as 

long-term solutions. 

Melatonin

Melatonin is a naturally occurring chemical in the body 

that is known to induce sleep. As one might expect, the 

release of melatonin in the body depends on retinal light 

exposure. Put simply, sunlight inhibits melatonin release 

while the lack of sunlight permits natural melatonin stores 

to be released, inducing the onset of sleep (Cardinali, Furio, 

Reyes, & Brusco, 2006; Skene & Arendt, 2006).

If administered properly, exogenous melatonin 

(melatonin not naturally made by the body) administration 

has been used to phase shift the sleep cycle to overcome 

shiftwork or jetlag (Melatonin, 2005; Skene & Arendt, 

2006). To overcome jetlag and shiftwork the onset of sleep 

either needs to be sped up or slowed down. When traveling 

eastwards or trying to sleep after working the night shift, 

administration of melatonin will speed up the onset of sleep 

and allow those affected to get onto the sleep cycle they 

desire. Melatonin can also be used to speed up the initial 

onset of sleep for people with insomnia. 

Sedatives

Sedatives have been commonly used as a 

countermeasure to insomnia within the general population. 

In fact, sedatives have been shown to be useful for inducing 

sleep at times when or in places that are not necessarily 

conducive to sleep (Caldwell & Caldwell, 2005). When 

considering the use of sedatives, care must be taken to 

choose the correct one. The available time for sleep must 

be considered, as sedatives with long half-lives (i.e., the 

time it takes to decay to half of the initial value), can cause 

performance decrements if a user is suddenly awoken for 

duty (Caldwell, 2005; Caldwell & Caldwell, 2005).

Antihistamines can also cause a sedative effect. 

However, like sedatives many antihistamines can produce 

drowsiness and decreased performance (Verster & 

Volkerts, 2004). Notably, all sedatives have detrimental 

effects on normal sleep architecture.

Alcohol

The use of alcohol as a countermeasure to insomnia is 

generally not recommended. While the consumption of 

alcohol before bedtime has been shown to speed up the 

onset of sleep, like other sedatives, alcohol is known to 

disrupt normal sleep architecture leading to feelings of 

fatigue and sleepiness upon awakening (Vitiello, 1997). 

Light Exposure

Exposure to bright light, a natural zeitgeber, has proven 

effective at reducing fatigue and improving circadian 

resynchronization. Recall that the light/dark cycle in 

humans plays an important role in synchronizing the 

circadian pacemaker (Duffy, Kronauer, & Czeisler, 1996). 

Not surprising then, the exposure to specific types of bright 

light (i.e., those that simulate the natural wavelengths seen 

in sunlight) may be used to phase delay the body’s 

circadian rhythm in some operational settings (Boivin & 

James, 2002) and may also help in overcoming the effects 

of jet-lag due to rapid scheduling changes (Wetterberg, 

1994; Caldwell, 2005; Revell & Eastman, 2005; Lack & 

Wright, 2007). In addition, there do not appear to be any 

age or gender differences in response to 3000 lux (Kripke, 

Elliott, Youngstedt, & Rex, 2007).

It has been shown that a single 4-hour exposure to light 

on the first day of the night shift can phase-delay the 

secretion of melatonin by 2-hours and thus delay the onset 

of sleepiness and fatigue and improve performance on the 

night shift (Dawson & Campbell, 1991). Dark glasses may 

also help avoid resynchronization to the natural circadian 

rhythm and allow workers to adapt more quickly to 

nighttime shiftwork (Eastman, et al., 1994; Yoon, Jeong, 

Kang, & Song, 2002). There is also some evidence that 

bright light exposure during the day may help with fatigue 

by promoting better nocturnal sleep for workers 

(Wakamura & Tokura, 2000). 

Unfortunately, while attempts to limit natural zeitgebers, 

such as exposure to the sun, can improve an individual’s 

ability to sleep, it is not without potential negative 

consequences (Lack & Wright, 2007). For example, while 

bright light therapy and the use of sunglasses during the 
15

day can help workers adjust, the body never fully adapts to 

shiftwork cycle (Lac & Chamoux, 2004). As a result, this 

can lead to chronic stress and fatigue, among other side 

effects. Still, the usefulness of strategically placed bright 

light has proven useful. 

Diet and Exercise

One aspect of good sleep hygiene is diet. While the 

reasons for insomnia in shiftworkers vary, in general 

differences in the timing of meals, the unavailability of 

well-balanced meals at night, and snacking all contribute to 

difficulties with sleeping (Atkinson & Davenne, 2007). 

That being said, people who suffer from insomnia should 

avoid large meals (Morin, et al., 2006) and foods with high 

sugar or caffeine contents before bedtime (Atkinson & 

Davenne, 2007).

Another aspect of good sleep hygiene is exercise. 

Exercise later in the day can increase the depth and length 

of sleep during the night (Caldwell, 1997) and when 

performed at night can further help to adapt to night 

shiftwork (Atkinson & Davenne, 2007).

It appears that the increase of body temperature from 

exercise is associated with a faster sleep onset (Stepanski & 

Wyatt, 2003). However, the timing of the exercise is 

important. Evidence suggests that exercise should be 

completed 5-6 hours before bedtime and avoided within 3 

hours of sleep (Morin, Hauri, et al., 1999; Morin, et al., 

2006). 

Enhancing the Sleep Environment

Effort should be made by the shiftworker to ensure that 

the sleeping environment is conducive to falling asleep. For 

example, blackout shades and comfortable earplugs or 

sound attenuating headphones will improve day-sleep. In 

addition, creating bedtime routine is also part of good sleep 

hygiene. Having a consistent bedtime and wake-up time 

may help in the treatment of insomnia (Caldwell, 1997; 

Morin, Hauri, et al., 1999) as the circadian rhythm 

eventually adapts to yield sleep at these times (Caldwell, 

1997). 

Education

While many of the above countermeasures can provide 

some relief from the adverse impact of shiftwork and 

improve adaptation, educating and increasing the 

awareness of the workforce can enhance their efficacy. In 

doing so, it is important to incorporate both the shiftworker 

and management in the education process, as each can 

make improvements to the shiftwork schedule that the other 

cannot. For example, the shiftworker can use their time offduty to help them prepare for their tasks (i.e., limiting the 

use of drugs, alcohol and caffeine, imposing regular 

bedtime rituals, and keeping to a consistent sleep-wake 

cycle), while management can make improvements to the 

work environment that will help employees cope with the 

demands of shiftwork.

Summary and Recommendations

• Early morning starts should be avoided and flexible work 

times should be encouraged

• Controlled exposure to known zeitgebers such as, 

sunlight, meal cues, and social cues can ensure that 

people are working during the circadian peak and are 

resting during the circadian trough.

• Bright light treatments pre-launch, during the mission 

and post-launch can facilitate adaptation.

• Current findings indicate that melatonin has shown little 

effect with facilitating adaptation in spaceflight. 

• Bright light should be used when necessary and can be 

used during the night shift to aid in circadian adaptation 

(Revell & Eastman, 2005). 

• A structured and regimented daily schedule adhered to 7 

days a week has also shown to help individuals maintain 

adaptation. 

• Dietary needs of shiftworkers should be satisfied with a 

cafeteria. 

• The sleep environment should be stimulating, pleasant, 

and inviting (Monk et al., 1996). 

• Shiftworkers should be educated on techniques to aid 

adaptation and coping strategies.

DO TRADITIONAL SECOND AND THIRD SHIFTS REQUIRE 

DIFFERENT DUTY AND REST PERIODS?

The scientific literature does not address this question 

directly; however, there are clear differences between the 

shifts with the night shift being of greatest concern due to 

accumulating sleep loss. While there are some individual 

differences, scientists seem to agree that the average person 

needs roughly 8 hours of sleep each night to be properly 

refreshed (Akerstedt, Hume, Minors, & Waterhouse, 1997). 

Although individuals do well with one night sleep loss, a 

decrease by just a few hours from the recommended 

amount to as little as five hours of sleep per night can cause 

serious performance decrements (Dinges, et al., 1997). 

Specifically, sleep restriction has been shown to impair 

vigilance and working memory (Belenky, et al., 2003; Van 

Dongen, Maislin, Mullington, & Dinges, 2003), increase 

the propensity to fall asleep (Guilleminault, et al., 2003; 

Carskadon & Dement, 1981; Banks, & Dinges, 2005), and 

generally lead to a general decrease in vigor and alertness 

(Dinges & Kribbs, 1991; Kleitman, 1963). 

More important, successive nights of sleep loss can lead 

to a condition known as sleep debt. As the name implies, 

sleep debt can be reduced through replenishing sleep, 

which must be accrued in order to obtain full recovery 

(Hardaway & Gregory, 2005). Even a loss of 1-hour of core 

sleep per night can lead to performance deficits over time 

as seen in Figure 5. However, consecutive nights of 2, 3, 

and 4 hours of sleep per night can lead to alarming 

performance deficits. 
16

Figure 5. Performance deficits as a result of days of 

cumulative sleep loss (Source: Department of Behavioral 

Biology, Walter Reed Army Institute of Research). 

Summary and Recommendations

• Performance and sleepiness is worst for the night shift, 

followed by the day and evening shifts.

• It is recommended that sleep debt be tracked regardless 

of the shift worked, although the night shift is of greatest 

concern.

CAN A BREAK IN DUTY DURING A SHIFT EXTEND THE DUTY 

PERIOD?

Extending the workday beyond 8 hours to 12 hours or 

more is not unique. Many occupations require long duty 

days including those involved in healthcare, emergency 

personnel like paramedics, firefighters and police officers, 

and many others. It is important to note that many of those 

involved in work beyond a normal duty day are involved in 

tasks that are either highly specialized or doing things that 

cannot be easily shared through shiftwork (Stampi, 1989). 

In other words, there may be few options available for 

shiftworkers in those situations.

There are other situations where management may have 

the flexibility to employ a variety of shift schedules. 

Unfortunately, the decision to move from a traditional 8-

hour shift to one of 12-hours or more is not an easy one 

since some have suggested that worker error rates may 

increase between 80% and 180% for some tasks when 

going from 8 hour to 12-hour shifts (Kelly & Schneider, 

1982). Moreover, even moving to a 12-hour shift involves 

more than just time spent working. Commuting time must 

also be taken into account whenever shift schedules are 

extended (Tepas & Monk, 1987). Assuming 1-hour 

commuting both directions and a 12-hour workday, 14 

hours of a normal 16-hour awake time (i.e., assuming 8 

hours of sleep) would be spent working leaving only a few 

hours for family and personal needs.

Nevertheless, the question is whether an organization can 

safely extend the duty day by incorporating a break in duty 

(i.e., napping)? The answer is yes; but proceed with 

caution.

While scientists agree that the average person needs 

roughly 8 hours of sleep each night to be properly refreshed 

(Akerstedt, et al., 1997), many workers simply do not get 

that much sleep daily. One supplement to normal core sleep 

is a nap (Dinges, Orne, Whitehouse, & Orne, 1987). Note 

that the operative term used was a “supplement,” not a 

“replacement” or “alternative” to core sleep, as naps do not 

eliminate the decline in cognitive performance and 

vigilance that occurs during normal circadian variation or 

sleep loss. Rather, naps merely moderate the adverse 

effects fatigue (Della Rocco, et al., 2000). That being said, 

the experts all agree that naps can be quite beneficial, 

particularly for individuals engaged in shiftwork (Dinges, 

Whitehouse, Orne, & Orne, 1988; Akerstedt, 1988).

While, in general, napping is viewed as a positive 

addition to any shiftworkers routine, a word of caution is 

required. Napping may have a negative effect on the quality 

and quantity of the subsequent main sleep period 

(Akerstedt & Torsvall, 1989). In other words, many 

individuals have experienced difficulty getting to sleep at 

night following a longer nap during the day.

That being said, the literature has consistently reported 

that naps are particularly effective at improving 

performance during short- and long-term sleep deprivation. 

While long-term sleep deprivation is rarely seen in 

shiftworking populations, short-term sleep deprivation is 

observed, particularly where night shifts are included. In 

those populations, even a little sleep in the form of a nap 

can be effective at improving a variety of performance and 

behavioral measures (Dinges, et al., 1975).

So, how long does a nap need to be in order to be 

effective? There appears to be no clear consensus as to 

what constitutes a short, medium, or long nap and therefore 

most have avoided such classifications. Rather, the duration 

of a nap is only meaningful when considered relative to an 

individual’s sleep cycle. For example, napping up to, but 

not more than 45 minutes in bed typically allows an 

individual to wake up refreshed with some short-term gains 

(Rosekind, et al., 1996). This is because in most cases the 

sleep period has not exceeded lighter sleep stages (Stage I 

and II), so awakening is quick with few residual effects.

In contrast, individuals engaged in longer naps run the 

risk of waking up during one of the deeper stages of sleep 

(Stages III or IV), which often results in a temporary state 

of lowered arousal and performance decrements 

immediately upon awakening. This phenomenon, referred 

to as sleep inertia, can persist for several minutes (Tassi & 

Muzet, 2000) and may produce a short-term (roughly 10-15 

minutes) decrement in performance, vigilance, and 

cognition (Della Rocco, Comperatore, Caldwell, & Cruz, 

2000).

Although the impact of sleep inertia can be quite 

debilitating, the effects can be reduced or even eliminated 

by keeping the nap length short, reducing prior sleep debt, 

waking during non-slow wave sleep, and avoiding waking 

during a circadian trough (Della Rocco, et al., 2000; 

Driskell & Mullen, 2005; Tassi & Muzet, 2000). Additional 

measures to counteract against sleep inertia include 

engaging the worker (e.g. by initiating a conversation) or 

exposing the worker to pink noise (intermediate between 

white noise and red noise) immediately upon awakening 

(Tassi, et al., 1992).
17

Given the adverse impact sleep inertia poses, it is 

typically recommended that a long nap should be at least 

two hours in length. This would allow enough sleep time to 

complete an entire sleep cycle which lasts roughly 90-120 

minutes in adults plus a reasonable amount of time to 

overcome any effects of sleep inertia (Martin, 2004; 

Rosekind, et al., 1996).

While shorter naps of less than 45 minutes and naps of at 

least 2 hours are recommended, a nap of approximately 45 

minutes to two hours in duration should be avoided. A nap 

of this duration may force an individual to wake up while 

in slow wave sleep; thereby, exacerbating any effects of 

sleep inertia.

For practical purposes, most organizations are more 

interested in how long a nap must be in order to be 

effective rather than the short-term effects of sleep inertia. 

Not surprisingly, naps of 1-2 hours have been shown to 

significantly increase the level of alertness and to a lesser 

extent, the level of performance (Dinges, et al., 1988; 

Vgontzas, 2007) and memory (Tucker, et al., 2006). 

However, even naps as short as 10 minutes can be 

restorative (Brooks & Lack, 2006). Still others have 

demonstrated that brief naps of 15 to 20 minutes increase 

the alertness levels of workers and increase the accuracy in 

vigilance tasks (Purnell, Feyer, & Herbison, 2002; 

Takahashi, Nakata, Ogawa, & Arito, 2004).

Although short naps can be effective, there does appear 

to be a loose relationship between nap length and 

improvements in performance, vigilance, and cognitive 

tasks. In general, the longer the nap duration, the greater 

the benefits achieved by the nap. Typical of the few studies 

that have directly compared nap length, Della Rocco et al., 

2000 found that a one or two hour nap is more effective 

than a 30 minute nap which is more effective than a 15 

minute nap in improving alertness, with the one and two 

hour naps being statistically equivalent. These data suggest 

that the largest recuperation or relative payoff occurs in the 

early part of the nap (Buxton, 2003).

In perhaps one of the most ambitious studies of its kind, 

Mollicone and his colleagues (2007 & 2008) examined the 

effect of daily naps of 0.4 to 2.4 hours time in bed (TIB) 

following 10 days of sleep restriction of 4.2, 5.2, 6.2, and 

8.2 hours TIB in 93 healthy adults. Remarkably, there did 

not appear to be any differences in sleep efficiency between 

sleep obtained in one episode at night or split-sleep 

episodes divided among nighttime sleep and a daytime nap 

(Mollicone, Van Dongen, and Dinges, 2007). Perhaps more 

important, splitting sleep up did not negatively affect 

daytime psychomotor vigilance, speed, and accuracy of 

cognitive throughput, or subjective sleepiness. Instead, 

performance appeared to be related to total TIB. This 

suggests that split sleep schedules that include naps during 

the day are reasonable when operational tasking requires 

nighttime shiftwork (Mollicone, Van Dongen, Rogers, and 

Dinges, 2008).

Summary and Recommendations

• Extending the shift from 8 hours to 12 hours or more 

will reduce the ability of workers to interact socially 

outside of the work environment and can interfere with 

the ability to get necessary core sleep.

• Although strategically placed naps are not a replacement 

for core sleep they can moderate performance deficits 

associated with the circadian trough.

• Naps have the potential to be more effective during the 

night shift where deficits are typically more pronounced.

• A nap as short as 10 minutes can be restorative; however 

there is a positive relationship between nap length and 

performance.

• Longer naps of 1-2 hours should be avoided when there 

is a potential need to respond immediately upon 

awakening due to the effects of sleep inertia.

HOW MUCH DUTY TIME CAN BE EXTENDED WITH FOLLOW 

ON AND/OR ‘MAKE-UP’ REST?

Again, the literature does not address this question 

directly. Perhaps the better question is can duty time be 

extended by strategically placing naps and if so, how long 

do those naps have to be. Most studies have focused on two 

types of nap placements: prophylactic and supplemental. 

Prophylactic naps are those naps that occur before a sleep 

debt is incurred, whereas supplemental naps include those 

that take place once sleep loss has been accrued.

When thought of in these terms, prophylactic naps are 

generally preferred over supplemental naps since they 

occur “after a period of sufficient main sleep ... [and] 

before serious sleepiness/fatigue has set in” (Buxton, 

2003). In addition, prophylactic naps tend to be short in 

nature and produce more vigilance and performance 

improvements with less sleep inertia than supplemental 

naps (Buxton, 2003).

The timing of prophylactic naps in relation to 

anticipated sleep loss is very important as well, with earlier 

naps being more advantageous (Della Rocco, et al., 2000).

In fact, the earlier the nap is taken in relation to the 

anticipate sleep debt, the “more robust and longer lasting” 

the performance benefits (Dinges, et al., 1987). Consider, 

for example, a crewmember scheduled to work the night 

shift; a short nap just before reporting for duty would slow 

the onset of fatigue/sleepiness, thereby allowing the 

crewmember to exhibit greater alertness and safer 

performance than those who did not nap prior to working 

the night shift (Buxton, 2003; Petrie, Powell, & Broadbent, 

2004).

So, are there optimal times to nap? Indeed, for 

prophylactic naps, there are more optimal times to take 

naps than others and not surprising the preferred napping 

times are directly related to an individual’s circadian cycle. 

Recall that the circadian rhythm typically reaches a 

minimum between 3 a.m. and 6 a.m. and peaks somewhere 

between 2 p.m. and 8 p.m. (Waterhouse, et al., 2005). Just 

as performance cycles about the circadian rhythm so does 

sleepiness and fatigue. 
18

During the circadian trough a person’s natural inclination 

to sleep increases, their alertness level decreases, their 

cognitive abilities drop, and performance reaches a 

minimum (Nakano, et al., 2001). Sleeping outside of the 

circadian trough often results in reductions in total sleep 

time, less SWS and REM sleep, performance deficits, and 

frequent awakenings (Akerstedt, et al., 1997). Indeed, 

studies have shown that taking a nap during the circadian 

trough restores and improves performance and vigilance 

more than one taken during a non-trough time (Della 

Rocco, et al., 2000) and has the least impact on a person’s 

circadian rhythm (Takeyama, Kubo, & Itani, 2005). 

Figure 6. Temporal placement of nap: in relation to the 

circadian rhythm (Figure taken from Arthur, 2008)

Nevertheless, there is also tremendous pressure to sleep 

in the afternoon between 1 p.m. and 4 p.m. (Figure 6). 

Often referred to as the post-prandial dip, napping during 

the mid-afternoon has been shown to be more efficient and 

has more impact compared to naps taken at other times of 

the day. Consequently, it is often recommended that 

workers nap during the afternoon prior to working a night 

shift (Buxton, 2003). 

However, there are issues (i.e., sleep inertia) associated 

with awakening from a nap during the circadian trough that 

are not as evident during other times of the day. It appears 

that sleep inertia is significantly worse during the circadian 

trough than during the day (Scheer, Shea, Hilton, & Shea, 

2008), particularly when awakening from non-REM sleep 

(Silva, E. and Duffy, J., 2008). What’s more, the effects 

may be worse for older adults (Silva & Duffy, 2008). The 

circadian nature of sleep inertia has important ramifications 

for those in mission critical tasks that may require optimal 

performance immediately upon awakening.

In one particularly interesting meta-analysis, Driskell 

and Mullen (2005) examined 12 studies that investigated 

the use of naps as a countermeasure to fatigue and 

performance. Using a multiple regression, they were able to 

show that naps as short as 15 and 30 minutes could reverse 

the effects of sleep deprivation (i.e., return performance to, 

or exceed baseline measures) up to 6 hours while naps in 

excess of 4 hours sustained performance up to 10 hours. 

Naps of 1 and 2 hours sustained performance 7 and 8 hours, 

respectively. 

While Dirskell and Mullen’s meta-analysis is intriguing, 

others have seen substantially less improvement. For 

example, Brooks and Lack (2006) showed improvements of 

up to 125 minutes following a 20-minute nap and 155 

minutes following a 30-minute nap. Furthermore, Driskell 

and Mullen did not compare differences that may exist 

among shifts and/or time of day. It is possible that 

performance effects would be less during the circadian 

trough.

When working the night shift, it is recommended that 

employees take a prophylactic nap before work or early on 

into the shift. Surprisingly, recommendations regarding 

naps may vary according to the type of shift rotation. On a 

fast shift rotation of less than five days between shift 

changes, workers may benefit from a short nap during the 

early part of the night shift (Buxton, 2003; Dinges, 1995). 

However, if on a slow shift rotation (five or more days 

between shift changes) or on permanent nightshift, workers 

should avoid napping and keep any necessary naps brief so 

that their circadian cycles corresponds with nocturnal 

activity (Buxton, 2003).

Summary and Recommendations

• Exactly how much duty time can be extended with 

follow on and/or “make-up” rest is unknown. 

• Naps in anticipation of sleep loss have been shown to 

improve vigilance and performance.

• Naps just before reporting for the night shift are 

recommended to improve performance.

• Caution must be exercised when napping during the 

circadian trough, particularly if performance needs to be 

optimal immediately upon awakening, as sleep inertia is 

known to last longer during those times.

• Short naps can sustain performance up to 6 hours while 

longer naps may sustain performance as long as 10 

hours. Others have shown significantly less sustained 

improvement with shorter naps. 

• It is quite likely that the efficacy of the naps would be 

reduced during the circadian trough.

HOW MUCH ‘MAKE-UP’ REST IS NECESSARY FOR AN 

EXTENDED DUTY PERIOD?

While there is no evidence that individuals can store or 

“bank” sleep in anticipation of potential sleep loss 

associated with shiftworking (Stampi, 1989), the question 

is how long it takes to fully recover and if duty time can be 

extended. Here again, the scientific literature is unclear. For 

example, in a study performed by Akerstedt and Torsvall 

(1981), the sleep after the night shift was found to be 

shorter. Individuals also ranked the sleep after the night 

shift worse than other shifts. However, the night shift is not 

the only shift with problems. The length of the sleep after 

the morning shift was found to be similar in length to that 

of the sleep after the night shift. As age increases, more 

problems arose in connection with sleep length and sleep 

quality. Individuals who are considered morning active 

have difficulty adjusting to the day time sleep after that 

occurs after the night shift (Akerstedt & Torsvall, 1981). 

On the other hand, others have found that sleepiness, 

fatigue, and most performance deficits associated with 

sleep loss can be overcome with one or more nights of 8-10 

hours of quality sleep during the circadian trough (Tilley et. 

al., 1982; Dinges et al., 1997; Caldwell, & Caldwell, 1997; 

Sallinen, et al., 2008). Precisely how long recovery sleep 

needs to be in order to restore performance to baseline 

levels is dependent largely on the extent of the sleep debt 
19

with moderate to large debts requiring more than one 

recovery night (Sallinen, et al., 2008).

One thing that is known is that “payment of sleep debt” 

is not a one-for-one proposition. In other words, a 

shiftworker who works five consecutive nights averaging 7 

hours of sleep per night would accrue 5 hours of sleep debt 

(1 hour x 5 days). Rather than requiring 13 consecutive 

hours (8 hours + 5 hours to compensate for the sleep debt) 

of recovery sleep, 8-10 hours of sleep should be sufficient. 

In fact, extending the sleep period beyond 8-10 hours can 

actually produce a phenomenon referred to as “sleep 

drunkenness” – a condition of prolonged sleepiness similar 

to a hangover (Herschovitch, Stuss, & Broughton, 1980; 

Taub & Berger, 1973).

While from a physiological/performance point of view 

recovery from sleep debt associated with shiftwork may be 

accomplished in one night, social concerns and morale may 

require more time. After all, off-duty time involves more 

than simply recovering from the stress of work. Days off 

allow social goals to be achieved as well. Therefore, the 

number of consecutive working days should be kept to only 

five to seven. Where possible, the shift schedule should 

also permit time off during the weekend when most family 

members are also enjoying time away from work and 

school. To do otherwise risks marital problems including 

divorce (Tepas, et al., 1985).

Summary and Recommendations

• Recovery from sleep loss and fatigue associated with 

shiftwork can typically be completed with 8-10 hours of 

quality sleep during the circadian trough.

• The number of days required to recover from sleep debt 

is tied to the degree of debt accrued. Moderate to large 

debts require more than one recovery night to return to 

baseline levels.

• Efforts to extend recovery after the night shift to 2 or 

more days off including weekends is encouraged to 

ensure full recovery and facilitate social health.

HOW IS DUTY AND REST TIME TRACKED AMONG DIFFERENT 

INDUSTRIES?

Generally speaking, all shiftworkers seem to get a 

reasonable amount of time to attend to personal 

maintenance, which includes sleeping, exercise and social 

interactions. However, that is an ideal world. Do they really 

get 8 hours of sleep as required? Moreover, are the pre- and 

post-sleep activities really to attend to “personal 

maintenance” or is it spent on other duty related or 

administrative tasks? What’s more, can one equate the 

operations in different industries? Unfortunately, the 

answers to these questions are beyond the scope of this 

review. 

Creating a shiftwork schedule manually can be beneficial 

particularly when trying to adapt to changing task demands 

or to personalize schedules that accommodate annual leave 

and sick days. Unfortunately, larger and more complex 

schedules are very difficult to complete by hand and the use 

of algorithms becomes a necessity. 

Recently, the scientific community has seen a proliferation 

of scheduling tools, many of which are based on many of 

the techniques found in operations research (Ernst, Jiang, 

Krishnamoorthy, Owens, & Sier, 2004). While the tools 

vary from one to another, most of the models track 

performance and alertness across days, nights, weeks, and 

even months. Central to all models is the integration of 

work and days off with sleep duration, some measure of 

recovery during sleep, time awake, circadian phase, and 

performance. 

To address fatigue related concerns associated with shift 

work, several shift work-scheduling tools have been 

developed. While all offer some degree of assistance to 

those tasked with managing shift workers, differences do 

exist. To give the reader a sense of the variety of tools 

being used, a brief description of several will be presented 

below.

• TURNI1 - TURNI (Abbink, Fischetti et al. 2005)

was established for use by NS Reizigers, the 

Netherlands’ Railway system, which operates 

5,000 time-tabled trains per day with over 3,000 

drivers and 3,500 conductors in 29 crew depots. 

The purpose of TURNI is to assign drivers and 

conductors to shifts and trains while following 

federally mandated laws and allocating the 

‘good’ and ‘bad’ shifts fairly. This system adds in 

penalties for over-scheduling workers, multiple 

transfers, night shifts, and positioning shifts. The 

drawback of this system is that reassignment of 

duties due to disruptions is more difficult than 

with the manual distribution of duties. 

• FAST - The Fatigue Avoidance Scheduling Tool 

(Eddy and Hursh 2001; Eddy and Hursh 2006). 

FAST was developed for use by the U.S. Air 

Force (USAF) as the computer user interface for 

their Sleep, Activity, Fatigue, and Task 

Effectiveness (SAFTE) model. The FAST 

interface allows the military scheduler to predict 

the performance levels of pilots based on past 

history such as sleep prior to assignment. FAST 

also allows for the construction of an optimal 

schedule based on the pilot’s physiological 

condition and indicates when and if 

pharmacological aids will be needed to address 

fatigue. 

• What makes the FAST model particularly 

appealing is that it incorporates performance and 

alertness, sleep propensity and sleep intensity, 

equilibrium state, progressive sleep debt under 

extreme measures, and sleep timing. Recently the 

USAF has introduced the FAST interface to 

markets outside the military for potential use.

• ALTITUDE - ALTITUDE was developed for use 

by Air Transit in Canada, which operates 18 

aircraft and 1,200 flight legs per month with over 

1 Unfortunately, several of the names of the shift work scheduling 

tools used acronyms that were undefined in any of the literature 

reviewed.
20

1,000 personnel. This model determines the best 

flight itinerary for each fleet, any repositioning of 

aircrafts needed, the release of aircraft, and 

maintenance schedules. The model incorporates 

maximum and minimum workload, the minimum 

rest time between scheduled flights for pilots, the 

maximum number of overseas trips without any 

time off, and other government and labor union 

contract rules while avoiding deadheads and 

minimizing the number of flight legs per duty 

period. The crew schedule for each month is 

completed in about three days and has helped 

lower the company’s turnover rate (Desrosiers, 

Lasry et al. 2000). 

• CrewResource Solver - The CrewResource 

Solver (Yu, Pachon et al. 2004) was developed 

by Continental Airlines which operates over 345 

aircraft, 4,500 pilots and 1,100 flights daily. This 

model is composed of four modules: staffing, 

vacation, planning, and training. The staffing 

module incorporates pilot work hours based on 

contractual and government rules, identifies 

shortages and surpluses based on planned flight 

schedules and current staffing levels, and 

transfers or relocations of pilots based on their 

future flight schedule. The other modules 

incorporate vacation bids, training, available 

resources, new hires, and pilot availability. 

• DISSY - DISSY (Emden-Weinert, Kotas et al. 

2001) was developed for the European Union’s 

bus and tram system. This model incorporates 

federal laws, union contracts and company 

agreements on weekly rests, nightly rests and 

maximum driving times. There is also a social 

aspect incorporated into the model that allows 

drivers to express their preference for shift types, 

duty types and days of the week. This model 

evenly distributes working times, hard shifts 

(shifts with multiple transfers) and length of 

shifts. 

• ERG - ERG (Hare 1997) was developed for use 

by healthcare facilities and works with the 

ILOG’s Solver library. This model generates shift 

rotations that satisfy staffing requirements, allows 

for part-time or full-time shifts and is flexible 

enough to be used to staff a variety of 

departments. The constraints of the model 

include baseline staff levels, minimum and 

maximum consecutive days worked, minimum 

and maximum number of hours per shift and 

week, shift change restrictions and allowable 

shifts. 

Of these tools, the most appealing to commercial space 

transportation is likely to be the FAST. What makes FAST 

particularly appealing is that it was built and validated 

using actual data from the field and laboratory and is based 

on scientific models of sleep and cognitive performance. In 

that way, it incorporates performance and alertness, sleep 

propensity and sleep intensity, equilibrium state, 

progressive sleep debt under extreme measures, sleep 

timing, and the transmeridian phase shift. In addition, 

FAST is dynamic and allows the scheduler to view the 

manner in which performance changes over time. The 

scheduler also has the ability to view the variability of 

performance based on user percentile.

Summary and Recommendations

• Most industries that have established requirements use 

log books to track duty and rest time.

• Key measurements that are tracked include: consecutive 

duty time, rest between duty time, and cumulative duty 

time in a given work span.

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