MINE SAFETY

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Mine Worker Fatigue and

Circadian Rhythms

How biological clocks respond to light and darkness

By Max Martell

The mining industry often uses shift work 

schedules with the intention to have 

a productive working mine around the 

clock. However, one potential side effect 

of operating on a 24/7 basis can be the 

disruption of circadian rhythms, which results in worker fatigue — the mental state 

between wake and sleep.

Fatigue from shift work has the potential to decrease productivity and increase 

accidents in organizations where workers 

are overly fatigued (Dawson et al., 2000). 

In open pit mines fatigue-related accidents account for up to 65% of truck 

driving accidents alone (Schmidt, 2015). 

The detrimental effects of severe fatigue include more than loss of productivity and accidents. Several potential 

health effects are directly associated with 

fatigue, including trouble sleeping, decreased alertness, slower reaction time, 

and a weakened immune system. Workers 

suffering from fatigue also show a general 

decline in cognitive abilities, such as problem solving and working memory. More 

serious health consequences of long-term 

chronic fatigue may include diabetes, obe38 E&MJ • FEBRUARY 2018

sity, heart disease, and an increased risk of 

cancer (Kecklund and Axelsson, 2016). 

 Although one cause of fatigue is lack 

of adequate sleep, fatigue is not the 

same as sleepiness. Additionally, fatigue 

can result from the disruption of circadian rhythms, also known as the body’s 

“biological clock.” Circadian rhythms 

are highly affected by the natural cycle 

of daylight, and varying exposure to light 

causes a disruption. Hence, shift workers 

are inherently at risk of circadian disruption, with their working hours extending 

into the night or early morning. This disruption can cause us to have problems 

with getting enough sleep. Even one night 

of poor sleep — defi ned as either less 

than seven consecutive hours or frequent 

waking — can have negative effects on 

our health (Watson et al., 2015).

Light and Circadian Rhythms

Short wavelengths of light, which are 

mostly blue, are abundant in daylight. 

The human eye’s responsiveness to these 

short wavelengths is what causes the sensitivity of circadian rhythms to daylight 

(Figueiro et al., 2016). During the day, 

the plentiful bright, blue light creates 

alertness. At night, the lack of blue light 

signals the brain that it is time to sleep. 

 However, for night shift workers, working at night and sleeping during the day 

disrupts the normal day/night progression. 

Furthermore, artifi cial light sources such 

as shop lights and computer screens disrupt the natural cycle of light and dark after 

sunset. These light sources are often high 

in short wavelengths and attempt to mimic 

daylight, since that spectrum of light improves both visibility and alertness. This 

leads to a disruption in circadian rhythms 

by confusing the body into thinking it is 

still daytime. As a consequence, workers 

on the night shift can experience diffi culty 

sleeping after their shift. 

 We also face a risk of circadian disruption during the day. When we do not get 

enough blue light exposure throughout 

the day, our body does not fully wake up. 

Just as with the previous scenario, when 

our biological clocks do not align with 

the natural light cycle, the misalignment 

can delay our normal period of sleep and 

cause us to have trouble sleeping. 

 Underground miners are one group at 

high risk for circadian disruption. Not only 

do they often work shift schedules and 

long hours, they may spend the majority 

of the day underground, where artifi cial 

sources supply all of the light. Even with 

bright white lights, like LEDs and fl uorescents, it is very diffi cult to get light exposure equivalent to that of daylight while 

underground. On some underground mining equipment, amber colored lights fi lter 

out much of the blue light. The problems 

are made worse by the low refl ectivity of 

the environment, making the ambient 

light darker as it refl ects back very little 

of the available light. Consequently, much 

less light reaches the eye than normal. 

Making the problem worse still is the aging population of the mining workforce. 

As we age, our ability to see shorter wavelengths of light decreases, adding to the 
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challenge of getting enough light exposure while underground (Boyce, 2014). 

Measuring Fatigue

People experience fatigue to varying degrees, and fatigue-causing factors affect 

some more than others. A person’s age or 

general health might affect their sensitivity to fatigue. However, there are ways to 

measure fatigue, which can be a positive 

fi rst step in addressing it.

 Subjective Methods–Subjective methods for measuring an individual’s fatigue 

include surveys and self-questionnaires, 

such as the Fatigue Severity Scale (FSS). 

Although this scale does not exactly measure “severity,” it does help to evaluate 

the impact that fatigue has on a person’s 

daily life. In the same way, the Psychomotor Vigilance Test (PVT) determines the 

impact of fatigue by measuring reaction 

time and alertness, and the Karolinska 

Sleepiness Scale (KSS) assesses subjectively how sleepy a person feels. Thus, 

these methods give us a way to understand the problem on an individual level.

 Objective Methods–More direct methods, such as measuring a person’s core 

body temperature or taking blood samples 

to measure hormone levels, can determine if there is a disruption of circadian 

rhythms. However, these procedures are 

highly invasive and cannot easily be done 

in real time, making them impractical for 

measuring fatigue in a working mine. Saliva tests to measure the hormone melatonin, a common indicator for circadian 

rhythms, are somewhat more practical. 

Other methods seek to identify fatigue as it 

happens. These include facial recognition 

and eye-tracking technology, which monitor 

a worker’s apparent fatigue level. This approach can make workers uncomfortable, 

however, as they might consider it an invasion of privacy or disruptive to their work. 

Dealing with Fatigue

A more active way of addressing worker 

fatigue is to use a preventative method. 

To control worker schedules and help 

limit shift work and excessive overtime, 

mine operators can implement a fatigue 

management system. These types of systems may help to manage fatigue resulting from shift work by fi rst detecting and 

tracking it, and then making changes to 

either avoid or manage fatigue over time. 

 Yet for underground miners, even with a 

fatigue management system in place there 

is still the problem with lack of suffi cient 

light. In this case, a lighting intervention 

may help to prevent the disruption of circadian rhythms. Other workers facing a similar issue are those working in Antarctica, 

who spend the sunless winter months in 

dark, isolated areas where the only light is 

from artifi cial sources. Similarly, the crew 

aboard U.S. Navy submarines may spend 

months at sea without natural light while 
MINE SAFETY

40 E&MJ • FEBRUARY 2018 www.e-mj.com

utilizing strict shift work schedules. Both 

were able to successfully adopt a lighting 

solution using highly blue-enriched light 

sources to provide enough illumination 

and short-wavelength light throughout the 

day to reduce the misalignment of circadian rhythms (Najjar et al., 2014; Young et 

al., 2015), which also led to an increase 

in alertness and quality of sleep. 

 Using these lights during the night 

shift, however, would contribute to the 

disruption of circadian rhythms. Yet workers still need light to do their jobs. One 

potential solution is to use light sources 

with longer wavelengths of light falling in 

the red spectrum. Research has shown 

that red light can increase alertness and 

performance without impacting circadian 

rhythms (Figueiro et al., 2016).

 Such a lighting solution would be challenging to implement in a mine environment, and the exact nature of an intervention remains a subject for future research. 

However, improving the lighting conditions 

has the benefi t of making hazards more visible to miners, and NIOSH researchers are 

hopeful that it can also serve as an effective 

fatigue intervention in underground mines. 

Max J. Martell is a Mining Engineer working at the National Institute for Occupational Safety and Health’s Pittsburgh 

Mining Research Division. He can be 

reached at: (412) 386-5855; Email; 

MMartell@cdc.gov.

Disclaimer

The fi ndings and conclusions in this paper are those of the authors and do not 

necessarily represent the offi cial position 

of the National Institute for Occupational Safety and Health, Centers for Disease 

Control and Prevention.

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