DOT/FAA/AM-02/8

Crystal Cruz

Cristy Detwiler

Thomas Nesthus

Albert Boquet

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

May 2002

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, VA 22161.

A Laboratory Comparison of 

Clockwise and Counter-Clockwise 

Rapidly Rotating Shift Schedules, 

Part I. Sleep Office of Aerospace Medicine

Washington, DC 20591
N O T I C E

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.

i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-02/8 

4. Title and Subtitle 5. Report Date 

 

A Laboratory Comparison of Clockwise and Counter-Clockwise Rapidly 

Rotating Shift Schedules, Part I. Sleep 

May 2002 (rev. 11/1/11) 

6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

Cruz, C., Detwiler, C., Nesthus, T., and Boquet, A. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 

Oklahoma City, OK 73125 11. Contract or Grant No. 

 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, D.C. 20591 14. Sponsoring Agency Code 

 

15. Supplemental Notes 

This work was performed under Task AM-A-00-HRR-519. 

16. Abstract: 

INTRODUCTION. Many air traffic control specialists work relatively unique counter-clockwise, rapidly 

rotating shift schedules. Researchers recommend, however, that if rotating schedules are to be used, they 

should rotate in a clockwise, rather than a counterclockwise direction. Unfortunately, few studies have 

examined clockwise and counter-clockwise, rapidly rotating shifts. This study was designed to partially remedy 

this lack by examining the effects of both types of schedules on sleep duration, timing, and quality. 

METHODS. Participants (n=28) worked a week of day shifts (0800-1600), followed by two weeks of either a 

Clockwise (n=14) or Counter-Clockwise (n=14) shiftwork schedule, including early morning (0600-1400), 

afternoon (1400-2200), and midnight shifts (2200-0600). Participants recorded sleep data in daily logbooks 

and wore wrist activity sensors to provide an objective source of sleep/wake data. RESULTS. Both groups 

reported less sleep before the early morning shifts (MClockwise=5.1h; Mcounter-clockwise=5.5h) than before the afternoon 

shifts (MClockwise=7.5h; Mcounter-Clockwise=7.9h). The Clockwise group reported an average of 7.2h of sleep during the 

night before the midnight shifts and a nap during the day of 0.9h. In addition to a nighttime sleep of 6.0h, 

the Counter-Clockwise group also took a nap before the midnight shifts of 2.2h. Objective sleep data and 

subjective sleep quality ratings are also reported. DISCUSSION. While these data indicate that a longer 

concentrated sleep period is obtained prior to the midnight shift on the Clockwise rotation schedule, they do 

not fully support the hypothesis that a clockwise rotation will result in less sleep disruption. 

 

17. Key Words 18. Distribution Statement 

Document is available to the public through the 

National Technical Information Service 

Shiftwork, Sleep, Circadian Rhythms 

Springfield, Virginia 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 22 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

1

A Laboratory Comparison of Clockwise and Counter-Clockwise

Rapidly Rotating Shift Schedules, Part I. Sleep

Introduction

Debate exists in the shiftwork literature regarding the 

benefit of rotating versus fixed-shift schedules (Folkard, 

1992; Wedderburn, 1992; Wilkinson, 1992). There is 

little debate, however, that if shift schedules are to rotate, 

they should do so in a clockwise direction to maximize 

the time off between shifts and take advantage of the 

delaying properties of the circadian clock (Barton & 

Folkard, 1993; Czeisler, Moore-Ede, & Coleman, 1982; 

Folkard, 1989; Knauth, 1993). The primary arguments against the counter-clockwise rotation of shifts, 

especially for rapidly rotating schedules in which shifts 

change every two or three days, are that it results in 

greater disruption of circadian rhythms and shortened 

sleep periods as a result of reduced time off (Czeisler, 

Moore-Ede, & Coleman, 1982; Folkard, 1989; Rosa et 

al., 1990). Others (Akerstedt, 1990; Barton & Folkard, 

1993) concede, however, that despite the arguments 

against counter-clockwise rotations, there is very little 

empirical evidence to support this view. Instead, Turek 

(1986) asserts that circadian rhythms and the sleep-wake 

cycle will be similarly perturbed on either a clockwise 

or counter-clockwise shift rotation.

Given this discussion, it is interesting to note that 

Air Traffic Control Specialists (ATCSs) in the United 

States have successfully worked variations of the counterclockwise, rapidly rotating shift schedules since the early 

1970s. The most common and well-known of these 

schedules is the so-called 2-2-1 (Cruz & Della Rocco, 

1995) that involves working two afternoon shifts, followed by two morning shifts and one midnight shift. 

Summarizing laboratory and field research on the 2-2-

1 schedule, Della Rocco, Cruz, and Schroeder (1995) 

demonstrated a characteristic decline in sleep duration 

across the workweek. Specifically, sleep duration declined 

from around 8 hours before the two afternoon shifts to 

roughly 5-6 hours before the two morning shifts. This 

was followed by a nap of approximately 2.5-3.5 hours 

during the daytime quick-turn before the midnight 

shift. While the timing of sleep co-varied with the 

start of the shift in the laboratory, sleep times in the 

field appeared more stable. That is, the timing of sleep 

onset was consistent, and wake-up times shifted only 

to accommodate earlier shift start times. These data are 

in agreement with findings reported by Folkard (1989) 

regarding the difficulties associated with reduced sleep 

due to early start times for the morning shift. 

While the 2-2-1 counter-clockwise rotating schedule has been utilized by ATCSs in the U.S. for years, 

surprisingly few studies have examined the effect that 

advancing and delaying schedules have on sleep-wake 

patterns and performance. One such study, a survey 

conducted by Barton and Folkard (1993) of shiftworkers 

in five different industries, revealed that those working 

advancing (counter-clockwise) schedules reported poorer 

physical and psychological health, greater sleep disruption, more social and domestic disruption, and lower 

job satisfaction than those working delaying (clockwise) 

shift schedules. In a study of microelectronic factory 

workers, changes in sleep-wake patterns were studied 

as individuals moved from a counter-clockwise shift 

system to a clockwise shift system (Lavie, Tzischinsky, 

Epstein, & Zomer, 1992). The authors concluded that 

results for sleep-wake cycle and subjective measures of 

fatigue supported the clockwise rotation of shifts. Most 

of the results, however, were not statistically significant. 

Likewise, other studies supporting clockwise rotations 

have been confounded by concomitant changes in other 

aspects of the schedule design (Czeisler, Moore-Ede, & 

Coleman, 1982; Knauth & Kiesswetter, 1987). 

The question remains whether clockwise shift rotations result in better adaptation and performance as the 

literature would suggest, particularly in the safety-critical 

environment of air traffic control. Notably, many air 

traffic controllers (ATCs) in the U.S. Air Force (USAF) 

work a schedule that is, in many ways, the clockwise 

counterpart to the counter-clockwise 2-2-1 schedule 

worked by FAA ATCSs. The USAF schedule rotates every 

2 days through morning, afternoon, and night shifts 

and then provides 48 hours off in a 6-day workweek. 

Luna, French, Mitcha, and Neville (1994) examined this 

schedule with 14 USAF ATCs and found that diurnally 

oriented circadian rhythms were maintained. 

Indeed, while there are a few studies investigating 

the direction of shift rotation, there appear to be none 

that directly compared rapidly rotating advancing and 

delaying shifts. The purpose of the present study was to 
2

directly compare clockwise and counter-clockwise rapidly rotating schedules. Specifically, this study examined 

the effects these schedules had on sleep duration, sleep 

onset and awake times, sleep quality ratings, subjective 

ratings of sleepiness, and subjective ratings of mood/

activation. 

Methodology

To examine differences between rapidly rotating 

clockwise and counter-clockwise shift schedules, a 3-week 

protocol was designed using groups of 5 participants at 

a time. The direction of rotation was balanced such that 

the first group was assigned to the clockwise rotation; 

the second group was assigned to the counter-clockwise 

rotation, and so on. An array of sleep, subjective fatigue, 

performance, physiological, and biochemical data was 

collected. However, this paper will focus on only those 

variables related to sleep and subjective measures.

Participants

Participant Recruitment and Selection 

Participants were initially recruited and screened by a 

private contractor. Upon arriving at the Civil Aerospace 

Medical Institute (CAMI) for the first time, participants were briefed on the study protocol and signed 

the informed consent. They were then administered a 

number of screening tests, including a drug and alcohol 

use questionnaire, the Shipley Institute of Living Scale 

verbal and abstract reasoning tests (Zachary, 1986), the 

Digit Span Forward and Backward tests, and an FAA 

Class II medical examination (14 CFR, Part 67, Federal 

Aviation Regulation). Participants were required to score 

a minimum WAIS-R equivalent of 102 on the Shipley 

test to ensure that subject intelligence was similar to 

students entering the FAA Academy ATCS Nonradar 

Screen (Della Rocco, Milburn, & Mertens, 1992). In 

addition, participants were required to recall 4 digits on 

both the forward and backward digit span and to pass an 

FAA Class II medical exam. All participants were paid. 

Demographics and Group Assignment 

Thirty people between the ages of 20 and 55 (M

= 41.2 years) were recruited and screened from the 

general population to participate in the study. Two 

participants withdrew before completing the protocol. 

The remaining participants were assigned to either the 

clockwise (n = 14) or counter-clockwise (n = 14) rotation condition based on the order in which they were 

recruited. The clockwise rotation included 7 males and 7 

females with an average age of 40.6 years (sd = 9.4 yrs.), 

while the counter-clockwise rotation included 5 males 

and 9 females with an average age of 41.9 years (sd = 

9.0 yrs.). The majority of participants were caucasian 

(86%), married (57%), and had at least some college 

(89%). All were high school graduates, non-smokers, and 

light- or non-users of caffeine and alcohol. Participants 

represented a range of occupations from professional to 

trades with only one who was unemployed. 

Procedures and Apparatus

 Protocol

Participants in the study reported to the laboratory 

for 8 hours per day, 5 days per week, for 3 weeks. Participation during the first week (Monday-Friday) for 

both shift rotation conditions took place during the 

day shift (0800-1600). It began with an orientation 

to the laboratory and a detailed daily schedule for the 

study. Two one-time questionnaires were completed, 

a Morningness-Eveningness Questionnaire (Horne & 

Ostberg, 1976) and a biographical questionnaire. In 

addition, participants were given physiological monitoring devices and daily logbooks, and they were trained 

on their use. Finally, participants were trained on the 

Multiple Task Performance Battery (MTPB) and the 

Bakan Vigilance Task. The physiological monitors and 

all sensors (except the chest band) were worn 22.5 hours 

per day to accommodate a 1.5-hour break for showers and leisure activities. The only restriction was that 

the monitor should not be removed while sleeping or 

napping. The chest band sensor was only worn while 

working in the laboratory. The Bakan Vigilance Task was 

administered at the beginning and end of each workday, 

and the MTPB was performed three times each day. The 

daily protocol is presented in Table 1.

During the next two weeks, participants worked 

either the clockwise or counter-clockwise shift rotation 

schedule shown in Table 2. Note that the clockwise 

rotation allowed 24 hours off between each shift rotation and a 48-hour weekend before returning to work 

again. The counter-clockwise rotation allowed only 8 

hours off between each shift rotation and an 80-hour 

weekend before returning to work again. 

Participants were instructed to treat their participation in the study as a full-time job and to refrain from 

drinking alcohol or taking any drugs or medications 

during the course of the study, with the exception of 

ibuprofen, birth control pills, estrogen replacement, 
3

Table 2

Clockwise and Counter-Clockwise Shift Rotation Schedules

Clockwise Rotation Counter-Clockwise Rotation

Day Work Hours Hours Between Day Work Hours Hours Between

Monday 0600-1400 16 Monday 1400-2200 16

Tuesday 0600-1400 24 Tuesday 1400-2200 8

Wednesday 1400-2200 16 Wednesday 0600-1400 16

Thursday 1400-2200 24 Thursday 0600-1400 8

Friday-Sat 2200-0600 Thur-Friday 2200-0600

of physiological monitors, symptoms, and comments 

was collected in daily logbooks, modified from those 

developed by the National Aeronautics and Space 

Administration (NASA; Gander, Myhre, Graeber, Andersen, and Lauber, 1989). Measures that were utilized 

for analysis in this paper included sleep onset and awake 

times, sleep duration, reported awakenings, sleep quality 

ratings, Stanford Sleepiness Scale (SSS) ratings (Hoddes, Zarcone, Smythe, Phillips, & Dement, 1973), and 

Positive and Negative Affect Schedule (PANAS) ratings 

(Watson, Clark, & Tellegen, 1988). Sleep quality ratings 

(SQR) were obtained upon arising. SSS and PANAS 

ratings were obtained at 0hr, 2hr 45min, 4hr 45min, 

and 7hr 30min into the shift. SSS ratings were also collected for the drive home from each shift.

Physiological Monitor

The Series 2000 MiniloggerTM ambulatory physiological monitor (MiniMitterTM Co., Inc., P.O. Box 

3386, Sunriver, OR 97707) was used to measure the 

inter-beat-interval (IBI) of heart beats, core body 

temperature, wrist activity, and ambient light. Sensors 

included: 1) a PolarTM Chest Band for measuring IBI, 

and non-drowsy formula allergy medications such as 

ClaritinTM. In addition, subjects were instructed not to 

consume any caffeinated beverages or chocolate and were 

not allowed to eat bananas because of potential interference with the radioimmunoassays for cortisol. Diet was 

not otherwise controlled in the study. Participants were 

tested with the Intoxilyzer 9000TM breath alcohol test 

at the beginning of each workday to ensure compliance 

with the study protocol. None of the participants tested 

positive during the study. 

A final day of testing on Day 22 of the study included 

a final Bakan test session, checking in equipment, and 

filling out an exit questionnaire regarding the study 

experience and a group cohesiveness questionnaire. 

Bringing participants back to the laboratory on this 

final day was done to mitigate an end-of-study effect 

at the end of the previous week and to allow for physiological data collection on the weekend following the 

last shiftwork week. 

Logbooks

Information regarding sleep, sleep quality, mood, 

sleepiness, food and beverages, exercise, naps, removal 

Table 1 

Daily Experimental Protocol 

Time (in hours) Activity

Start End 

00:00 00:30 Download & initialize Miniloggers; subjective ratings; collect saliva 

00:30 01:00 Bakan Session 1 

01:00 02:30 MTPB Session 1 

02:30 02:45 Subjective ratings; collect saliva 

02:45 03:15 Break 

03:15 04:45 MTPB Session 2 

04:45 05:00 Subjective ratings; collect saliva 

05:00 05:30 Meal Break 

05:30 07:00 MTPB Session 3 

07:00 07:30 Bakan Session 2 

07:30 08:00 Download & initialize Miniloggers; subjective ratings; collect saliva 
4

2) a Yellow Springs InstrumentsTM flexible rectal temperature sensor, 3) a small wrist activity monitor, and 

4) an ambient light sensor. All of these channels were 

direct wired to the Minilogger. Temperature, activity, 

and light measures were recorded in 1-minute epochs.

Design and Data Analysis

The design of the study was a mixed-model, repeated 

measures design where schedule rotation represented the 

between-groups variable, and week, day, session, and/

or sleep period represented the within-groups variables. 

The majority of analyses utilized the General Linear 

Model for Repeated Measures procedure, in addition to 

post-hoc comparisons using the Bonferroni adjustment 

for multiple comparisons. In the case of missing data, 

values were replaced with the comparable shift data for 

each participant. For example, if the sleep quality rating 

was missing for a given participant on the first earlymorning shift in the first week of shiftwork, this rating 

was replaced with that participant’s rating for the first 

early-morning shift in the second week of shiftwork. 

Subjective Sleep Data 

Sleep duration was calculated as reported sleep 

duration minus reported awakenings of 10 minutes or 

longer. In addition, 21 corrections were made when 

reported sleep duration was clearly miscalculated based 

on the reported sleep onset and awake times (e.g., when 

subjects miscalculated sleep duration by 1 or 2 hours). 

Daily logs from all 28 participants were included in the 

analysis, with only 1 missing data point during the 2 

weeks of shiftwork. 

Actigraphy Sleep Data 

Data from the wrist activity sensor were scored utilizing the Action-WTM version 2.0 software. Bad data, including missing data due to removal of the wrist monitor, 

as well as sensor or battery failure, were removed from 

each record. These times were verified using logbook 

reports and logged information regarding known failures 

of the device. The “auto set down interval” function was 

used as an initial sleep-scoring tool based on the ColeKripke (1988) re-scoring algorithm. “Down times” were 

then altered as necessary so that work times were not 

incorrectly scored as sleep, and sleep periods that were 

separated by brief awakenings were combined. Records 

from 6 participants could not be scored. Four of these 

records contained too much missing data due to device 

failures. One record could not be matched reliably to 

self-reported “logger off” times, leaving the scoring open 

to excessive error. Another record seemed to suffer from 

a mix of problems, including subject non-compliance, 

two failed/broken sensors, dead batteries, and mixed up 

channel plug-ins. Of the 22 records that were included in 

the analysis, there were 13 missing sleep periods during 

the 2 weeks of shiftwork. 

Subjective Ratings 

Responses to the SQR were re-scored so that high 

scores on each question indicated good sleep. Responses 

to all 4 questions were summed for a total SQR with 

a range of possible scores from 4 to 20. There were 3 

missing sleep quality ratings. Although SSS ratings only 

ranged from 1 to 7, these were analyzed with MANOVA 

because of a lack of non-parametric methods for analyzing mixed-model designs. There were 41 missing values 

for the SSS ratings, which represented 2.9% of the 

1400 ratings included in the analysis. About half of the 

missing ratings were for the end of the shift or the drive 

home. Scores for the negative affect (NA) and positive 

affect (PA) portions of the PANAS were calculated by 

summing the ratings for all the NA and PA adjectives. 

There were 5 missing PA and NA scores out of 1,120 

ratings (0.4%). 

Results

The results of the analyses for the following dependent measures are presented: subjective sleep duration, 

scored sleep duration, SQR, SSS, and PANAS ratings. 

Each of the dependent measures was assessed in terms 

of potential sleep/wake cycle disruption associated with 

clockwise and counter-clockwise shift rotation schedules. 

Descriptive statistics for each variable are included in 

Appendix A. To ensure that the groups were not different at baseline, statistical comparisons of the Day 4 

(Thursday) data were calculated for each measure. These 

analyses revealed no significant findings, indicating 

that the 2 groups were not different from each other at 

baseline. Likewise, the average WAIS-R score for those 

in the clockwise condition (M = 109.5, sd = 4.4) was not 

statistically different from those in the counter-clockwise 

condition (M = 111.1, sd = 6.0).

Self-reported Sleep Duration (Logbooks)

A 2 (week) x 6 (sleep period) x 2 (rotation condition) 

between-groups, repeated measures MANOVA was conducted on subjective sleep duration for the sleep periods 
5

before the 5 shifts and the first recovery night of sleep 

following each workweek. To accommodate the mix of 

night and naptime sleep obtained before the midnight 

shift, the night sleep obtained by the counter-clockwise 

group before the second early-morning shift (Mweek2 = 

5.8h; Mweek3 = 6.1h) was combined with the nap before 

the midnight shift (M = 2.2h for both weeks). Likewise, 

the night sleep obtained by the clockwise group before 

the midnight shift (Mweek2 = 7.0h; Mweek3 = 7.4h) was 

combined with the daytime nap before the midnight 

shift (Mweek2 = 1.1h; Mweek3 = 0.6h). The analysis revealed 

only a significant main effect for sleep period, F(5, 22) 

= 52.5, p = .000 (Figure 1). Post-hoc comparisons using 

a Tukey critical value revealed that sleep duration before 

the two early-morning shifts (M = 5.0h and 5.6h) was 

significantly shorter than before the two afternoon shifts 

(M = 7.9h and 7.5h), the midnight shift (M = 8.1h), or 

the recovery sleep period following the workweek (M = 

8.4h). In addition, sleep duration before the first earlymorning shift was significantly shorter than before the 

second early-morning shift. Subsequent analyses revealed 

a significant effect for age, F(1,26) = 7.2, p = .013, indicating that participants under 40 (M = 6.8h) received 

significantly less sleep than those over 40 (M = 7.4h). 

Analysis of the full model with age as a covariate did 

not result in different results than those reported earlier.

Scored Sleep Duration (Actigraphy)

A 2 (week) x 6 (sleep period) x 2 (rotation condition) 

between-groups, repeated measures MANOVA was 

conducted on sleep duration data scored from wrist 

actigraphy for the sleep periods before the 5 shifts and 

the first recovery night of sleep following each workweek. Again, night and naptime sleep periods before 

the midnight shift were combined, so that the night 

sleep obtained by the counter-clockwise group before 

the second early-morning shift (Mweek2 = 6.5h; Mweek3 = 

6.6h) was combined with the nap before the midnight 

shift (Mweek2 = 2.8h; Mweek3 = 2.9h). In addition, the night 

sleep obtained by the clockwise group before the midnight shift (Mweek2 = 7.5h; Mweek3 = 8.2h) was combined 

with the nap before the midnight shift (Mweek2 = 1.7h; 

Mweek3 = 1.3h). The results revealed only a significant 

main effect for sleep period, F(5, 16) = 40.9, p = .000 

(Figure 2). Post-hoc comparisons revealed that sleep 

duration before the 2 early-morning shifts (M = 5.6h 

and 6.5h) was significantly shorter than before the 2 

afternoon shifts (M = 8.2h and 8.0h), the midnight shift 

(M = 9.3h) and the recovery sleep period following the 

workweek (M = 8.6h). In addition, sleep duration before 

the first early-morning shift was significantly shorter than 

before the second early-morning shift, and sleep before 

the second afternoon shift was significantly shorter than 

the combined sleep before the midnight shift. Although 

sleep duration for the scored sleep was generally longer 

(by an average of 36 minutes) than subjectively reported 

sleep duration, the pattern of sleep duration was similar, 

and the statistical results were the same. 

Subsequent analyses revealed a significant effect for 

age, F(1, 20) = 6.8, p = .017, indicating that participants under 40 received significantly less sleep (M = 

7.3h) than participants over 40 (M = 8.1h) as found 

with the reported sleep durations from the logbooks. 

A reduced sample size and unequal representation of 

age and rotation condition (n = 4 clockwise/under 40; 

n = 5 clockwise/over 40; n = 7 counter-clockwise/under 

40; n = 6 counter-clockwise/over 40) due to missing 

data may have resulted in less power for the analysis of 

the full model with age as a covariate. As a result, the 

main effect for sleep period was not significant with 

the MANCOVA analysis. Examination of mean sleep 

duration by rotation condition and age for each sleep 

0

2

4

6

8

10

EM1 EM2 A1 A2 M R

Shift

Hours of Sleep

Figure 1. Subjective Sleep Duration

0

2

4

6

8

10

EM1 EM2 A1 A2 M R

Shift

Hours of Sleep

Figure 2. Scored Sleep Duration Figure 1. Subjective Sleep Duration Figure 2. Scored Sleep Duration

EM-Early Morning shift A-Afternoon shift, M-Midnight shift, R-Recovery night
6

period, however, revealed that both age groups within 

both rotation conditions received substantially less sleep 

before the 2 early-morning shifts than before any other 

shift in the schedule (See Table A3 in Appendix A).

Sleep Onset and Wake Up Times

A 2 (week) x 5 (sleep period) x 2 (rotation condition) 

between-groups, repeated measures MANOVA was 

conducted on sleep onset and wake up times from the 

scored wrist activity data for the sleep periods before 

the 2 early-morning and afternoon shifts and the first 

recovery night of sleep following each workweek. The 

results for sleep onset time revealed a significant main 

effect for sleep period, F(4, 17) = 16.0, p = .000. Sleep/

work cycles for the clockwise and counter-clockwise 

shift rotations are presented in Figures 3 and 4, with the 

last 3 days of the day shift week and the average sleep 

times for both shiftwork weeks. Post-hoc comparisons 

revealed that sleep onset before the first early-morning 

shift (M = 22:59) was significantly later than before 

the second early-morning shift (M = 22:12). Sleep 

onset time before the second early-morning shift was 

significantly earlier than before the afternoon shifts and 

recovery sleep period (M = 23:33, 23:53, and 23:45). In 

Sleep Work

D

D

D

Off

Off

EM1

EM2

A1

A2

M

Off

 6 p.m. 6 a.m. 6 p.m. 

Figure 3. Sleep/Work Cycle for Clockwise Rapidly Rotating Shift Schedule

D-Day shift, EM-Early Morning shift, A-Afternoon shift, M-Midnight Shift
7

addition, sleep onset before the first early-morning shift 

was significantly earlier than before the second afternoon 

shift. The MANOVA for wake up time also revealed a 

significant main effect for sleep period, F(4,17) = 47.4, 

p = .000. As expected, post-hoc comparisons revealed 

that awake time was significantly earlier before the two 

early-morning shifts (M = 04:39 and 04:40) than before 

the afternoon shift and recovery sleep periods (M = 07:53, 

07:54, and 08:34). Subsequent analyses indicated that 

there were no age effects for sleep onset or awake time.

Napping

Table 3 presents a cross-tabulation of the frequency 

of naps taken before the midnight shifts in the study. 

Clearly, a much larger proportion of individuals in the 

counter-clockwise rotation (79%) took a nap before the 

midnight shift in week 2 than individuals in the clockwise 

rotation (57%). This pattern was even more exaggerated 

in week 3, with 93% of the counter-clockwise condition 

taking a nap before the midnight shift, compared with 

only 36% of the clockwise rotation. Of those who took 

Sleep Work

D

D

D

Off

Off

A1

A2

EM1

EM2

M

Off

6p.m. 6a.m. 6p.m.

Figure 4. Sleep/Work Cycle for Counter-Clockwise Rapidly Rotating Shift Schedule 

D-Day shift, EM-Early Morning shift, A-Afternoon shift, M-Midnight shift 
8

naps, average reported nap length in the clockwise condition was 1.9h and 1.8h for weeks 2 and 3, respectively. 

And, of those who took naps in the counter-clockwise 

condition, the average reported nap length was 2.8h 

and 2.3h for weeks 2 and 3, respectively. 

Sleep Quality Ratings

A 2 (week) x 5 (sleep period) x 2 (rotation condition) between-groups, repeated measures MANOVA 

was conducted on SQR for the sleep periods before 

the 2 early-morning and afternoon shifts and the first 

recovery night of sleep following each workweek. The 

results revealed a significant main effect for sleep period, 

F(4,23) = 8.1, p = .000 (Figure 5). Post-hoc comparisons 

indicated that sleep quality ratings for the sleep period 

before the first early-morning shift (M = 12.6) were 

significantly lower than those for the sleep periods before 

the afternoon shifts and the recovery sleep (M = 14.8, 

14.6, and 15.8). Sleep quality ratings for the second 

early-morning shift (M = 13.8) were significantly lower 

than the recovery sleep period (M = 15.8).

Stanford Sleepiness Scale Ratings

A 2 (week) x 5 (shift) x 5 (rating) x 2 (rotation condition) between-groups, repeated measures MANOVA 

was conducted on SSS ratings over the workday and for 

the drive home for the 2 shiftwork weeks. The results 

revealed a significant shift by rating interaction, F(16, 11) 

= 7.6, p = .001. This relationship is depicted in Figure 6 

and shows that ratings of sleepiness increased across the 

workday for all shifts but particularly so for the midnight 

shift. Tukey multiple comparisons were conducted for 

ratings on the midnight shift and for ratings at the 

beginning, end, and drive home from each shift. On 

the midnight shift, sleepiness was significantly higher 

at the end of the shift (M = 5.6) than any of the other 

ratings, which ranged from 2.6 to 4.5. In addition, the 

third rating of the shift (M = 4.0) and the drive home 

ratings (M = 4.5) were higher than the first 2 ratings 

of the midnight shift (M = 2.6 and 3.0). Ratings at the 

beginning of the first early-morning shift (M = 3.2) were 

significantly higher than at the beginning of either of 

the afternoon shifts (M = 2.0 and 2.1). SSS ratings were 

significantly higher at the end of the midnight shift (M

= 5.6) than at the end of any of the other shifts (M = 

3.0, 3.6, 3.8, and 3.9). In addition, ratings were higher 

on the drive home from the midnight shift (M = 4.5) 

than on the drive home from any of the other shifts (M

= 2.9, 3.0, 3.1, and 3.1).

A reduced model (2 weeks x 5 ratings x 2 rotation 

conditions) was subsequently analyzed for SSS ratings 

on the midnight shift to further investigate possible relationships on that shift. Results of the analysis revealed 

a significant week by rating by rotation condition interaction, F(4,23) = 2.8, p = .037. Simple effects analysis 

revealed that individuals in the counter-clockwise rotaTable 3.

Frequency of Napping Before the Midnight Shift

Week 2 Week 3

Rotation Condition Nap No Nap Nap No Nap

Clockwise 8 6 5 9

Counter-Clockwise 11 3 13 1

0

5

10

15

20

EM1 EM2 A1 A2 R

Shift

Figure 5. Sleep Quality Ratings

EM1-1st early-morning shift M2-2nd

early-morning shift A1-1st afternoon

shift A2-2nd afternoon shift M-midnight

shift R-Recovery night
9

0

1

2

3

4

5

6

EM1 EM2 A1 A2 M

Shift

SSS Rating

Rating 1

Rating 2

Rating 3

Rating 4

Rating 5

Figure 6. SSS Ratings. Shift by Rating Interaction

EM1-1st early-morning shift EM2-2nd early-morning shift A1-1st afternoon shift A2-2nd

afternoon shift M-midnight shift

0

1

2

3

4

5

6

7

Week 1 Week 2

Shiftwork Week

SSS Rating

Clockwise Counter-Clockwise

Figure 7. SSS Ratings for the End of the Midnight Shift

tion reported significantly higher ratings at the end of 

the midnight shift during the first week of shiftwork 

(M = 6.1) than individuals in the clockwise rotation 

(M = 4.9), F(1,26) = 4.8, p = .038. There was no 

difference between rotation conditions, however, 

at the end of the midnight shift during the second 

week of shiftwork (Figure 7).

Positive and Negative Affect Schedule

A 2 (week) x 5 (shift) x 4 (rating) x 2 (rotation 

condition) between-groups, repeated measures 

MANOVA was conducted on negative affect (NA) 

and positive affect (PA) ratings over the workday 

for the 2 shiftwork weeks. Results of the NA ratings 

revealed no significant effects. On the other hand, 

results of the PA ratings revealed main effects for 

Shift, F(4,23) = 10.6, p = .000, and Rating, F(3,24) 

= 12.1, p = .000. Post-hoc comparisons for Shift 

(Figure 8) indicated that ratings of PA on the first 

afternoon shift (M = 21.8) were significantly higher 
10

0

5

10

15

20

25

EM1 EM2 A1 A2 M

Shift

PA Score

Figure 8. Positive Affect Ratings by Day

0

5

10

15

20

25

1 2 3 4

Rating

PA Score

Figure 9. Positive Affect Ratings by EM1-1 Rating Time st early-morning shift EM2-2nd early-morning shift 

A1-1st afternoon shift 

A2-2nd afternoon shift M-midnight shift

than ratings of PA on the early-morning shifts (M = 

19.0 and 19.1) or the midnight shift (M = 17.2). In 

addition, PA ratings on the second afternoon shift (M

= 20.5) were higher than those on the midnight shift. 

Post-hoc comparisons for Rating (Figure 9) indicated 

that the first PA rating of each shift (M = 21.8) was 

significantly higher than ratings 2, 3, or 4 (M = 19.8, 

19.6, and 17.0, respectively). In addition, PA ratings 

at the end of each shift were significantly lower than 

ratings 1, 2, or 3.

Discussion

This study represents one of the first experimentally 

controlled investigations of direction of rotation in 

rapidly rotating shift schedules. The purpose of this 

paper was to directly compare the sleep and subjective 

ratings of individuals working clockwise and counterclockwise, rapidly rotating shift schedules to test the 

hypothesis that the clockwise rotation results in better 

outcomes. Put simply, rotation condition did not play 

a significant role in the results of any of these measures. 

Instead, results of the study indicated that sleep duration, timing, and quality were dependent only on sleep 

period and that subjective sleepiness was dependent on 

both shift and rating time. 

Sleep Measures

Results of the study were similar to past research on 

the 2-2-1 (Della Rocco, Cruz, & Schroeder, 1995) and 

other schedules (Folkard, 1989) indicating that sleep 

duration is dramatically reduced before early-morning 

shifts, compared with afternoon shifts. It was expected 

that individuals in the clockwise condition would obtain more sleep before the first early-morning shift than 

those in the counter-clockwise condition because of the 

quick-turn-around in the counter-clockwise rotation. 

The truth, however, was that the first early-morning 

shift worked by the clockwise group suffered from a 

“weekend effect,” as participants continued to go to bed 

as late as they had all weekend and were forced to get 

up early for the early-morning shift, thereby reducing 

sleep duration. Indeed, both groups reported about 5 

hours of sleep before the first early-morning shift in 

their respective rotations. 

The direct comparison of clockwise and counterclockwise rotations led us to view the sleep obtained 

before the midnight shift differently than in the past. 

Specifically, it is true that the clockwise rotation has 

an “extra” night of sleep before the midnight shift; 

however, this night of sleep is followed by a full day of 

time spent awake for the most part before participants 

go to work on the midnight shift. On the other hand, 

sleep obtained before the second early-morning shift in 

the counter-clockwise rotation is followed by a day of 

work and a fairly significant nap taken by the majority 

of people working the schedule. In the past, we would 

have focused only on the nap before the midnight shift. 

Now, it is clear that the night of sleep, while reduced 

due to the early-morning shift, still contributes to the 

sleep obtained before the midnight shift and that it is 

unfair to view the nap before the midnight shift as the 

only sleep period obtained before that shift. Indeed, our 

findings indicate that, when naps were combined with 
11

the major sleep period, there was no difference between 

the rotation conditions in the amount of sleep obtained 

before staying up all night. 

The effect of the entire workweek might also be 

considered in terms of the amount of sleep obtained on 

the first night of sleep following the midnight shift (i.e., 

the recovery sleep period). Both groups obtained more 

than 8 hours of sleep during the recovery sleep period. 

This amount of sleep was not significantly longer than 

those observed before the afternoon shifts and was not 

statistically different between the clockwise and counterclockwise conditions, which indicates that direction of 

rotation did not differentially affect fatigue or sleep loss. 

While tolerance to shiftwork is generally considered 

to decline with increasing age (Akerstedt & Torsvall, 

1981; Monk & Folkard, 1985), one interesting finding 

from this study was that participants over 40 received 

significantly more sleep during the shiftwork weeks than 

participants under 40. Although there was no significant 

effect of age in the scored sleep onset and wake up time 

data, this may have been due to reduced power in the 

design because of reduced and unequal samples. An 

examination of the means for rotation condition and 

age, however, revealed that the differences in sleep duration appeared to be due to later bedtimes before the 

early-morning shifts and earlier wake-up times before 

the afternoon shifts and on the recovery sleep periods 

for the under-40 age group. 

Subjective Ratings

Subjective ratings, like the results of sleep duration 

and timing, revealed no differences between the rotation 

conditions. Instead, early-morning shifts, particularly the 

first one of the workweek, were once again identified 

as problem areas. Sleep quality was significantly lower 

before the first early-morning shift than before the 

evening or recovery sleep periods. In addition, ratings 

of sleepiness were significantly higher at the beginning 

of the first early-morning shift than at the beginning 

of the afternoon shifts. And, positive affect was significantly lower on the early-morning shifts than on the 

first evening shift. 

The midnight shift poses particular problems for 

maintaining performance and alertness during the night 

because it requires people to work against the biological clock (Cruz & Della Rocco, 1995; Della Rocco & 

Cruz, 1995). As expected, participants reported being 

significantly sleepier at the end of the midnight shift 

and on the drive home from the midnight shift than 

on any other shift. The average rating for the end of the 

midnight shift corresponded most closely to the scale 

response, “Sleepy, woozy; prefer to be lying down; fighting sleep,” while the average rating for the drive home 

from the midnight shift corresponded most closely to 

the scale response, “Foggy; slowed down; beginning to 

lose interest in remaining awake.” These results clearly 

demonstrate the need for effective counter-measures 

for the sleepiness experienced on the midnight shift 

and during what can be very dangerous, early morning 

rush-hour traffic.

In addition, analysis of sleepiness ratings focusing 

on the midnight shift revealed the only evidence in 

these data to indicate that the 2 conditions differed. 

Specifically, individuals in the counter-clockwise rotation 

reported being sleepier at the end of the midnight shift 

than individuals in the clockwise rotation in the first 

week of shiftwork. The fact that this difference disappeared in the second week of shiftwork is likely because 

the counter-clockwise group obtained an average of 20 

minutes more sleep before the midnight shift in the 

second week of shiftwork than in the first. This sleep 

was obtained not during the nap before the midnight 

shift but, rather, during the night before the second 

early-morning shift. This may indicate that in as short 

as 2 weeks of shiftwork, individuals began to incorporate 

coping strategies in the counter-clockwise rotation to 

increase sleep and thereby reduce their sleepiness on 

the midnight shift. 

Summary

The results from this study conflict with the bulk 

of the literature regarding direction of shift rotation, 

but support more recent suggestions that there may be 

fewer differences in these kinds of schedules than was 

once thought (Tucker, Smith, Macdonald, & Folkard, 

2000). They also support the theoretical work reported 

by Turek (1986) some years earlier that sleep patterns and 

circadian rhythms may be similarly affected by clockwise 

and counter-clockwise shift rotations. It is also important 

to note that most of the literature regarding direction 

of shift rotation has involved more slowly rotating shift 

schedules than those investigated in this study. While 

individual results of a non-significant nature generally 

fail to reveal or add much to the scientific understanding 

of a relationship, evidence is growing that direction of 

rotation, particularly in rapidly rotating shift schedules, 

does not affect outcomes such as sleep and subjective 
12

ratings of fatigue. Replications of this kind of research 

in the laboratory and in field research are needed to 

make reliable conclusions with regard to direction of 

shift rotation.

While the findings from this study are relevant to 

real-world shiftwork scheduling practices, one major 

limitation of this study is that it only represents shortterm adaptation to the clockwise and counter-clockwise 

rotation schedules investigated. It is not clear that sleep 

patterns would be similar for experienced shiftworkers, 

although the results for the counter-clockwise condition 

are consistent with the results obtained in other studies 

of FAA air traffic controllers in the field (Cruz & Della 

Rocco, 1995; Cruz, Della Rocco, & Hackworth, 2000). 

Specifically, the results from this study indicate that the 

counter-clockwise shiftwork schedules currently in use 

by air traffic controllers in the U.S. would not likely be 

improved by reversing the direction of rotation. In addition, it is clear that two major problem areas in both 

clockwise and counter-clockwise rapidly rotating shift 

schedules are early-morning and midnight shifts—areas 

that should receive more study in the future. 

References

Akerstedt, T. (1990). Psychological and psychophysiological effects of shift work. Scandinavian Journal 

of Work Environmental Health, 16 (1), 67-73.

Akerstedt, T. & Torsvall, L. (1981). Shiftwork. Shiftdependent well-being and individual differences. 

Ergonomics, 24 (4), 265-73.

Barton, J. & Folkard, S. (1993). Advancing versus delaying shift systems. Ergonomics, 36 (1-3), 59-64.

Code of Federal Regulations, Medical Standards and Certification, 14CFR67, Washington, DC: USGPO.

Cole, R. & Kripke, D. (1988). Progress in automatic 

sleep/wake scoring by wrist actigraph. Sleep Research, 17, 331.

Cruz, C. & Della Rocco, P. (1995). Sleep patterns in air 

traffic controllers working rapidly rotating shifts: A 

field study (DOT/FAA/AM-95/12). Washington, 

DC: Federal Aviation Administration, Office of 

Aviation Medicine.

Cruz, C., Della Rocco, P., & Hackworth, C. (2000). Effects of quick rotating shift schedules on the health 

and adjustment of air traffic controllers. Aviation, 

Space, and Environmental Medicine, 71 (4), 400-7.

Czeisler, C., Moore-Ede, M., & Coleman, R. (1982). 

Rotating shift work schedules that disrupt sleep are 

improved by applying circadian principles. Science, 

217, 460-3.

Della Rocco, P. & Cruz, C. (1995). Shiftwork, age, and 

performance: Investigation of the 2-2-1 shift schedule 

used in air traffic control facilities 1.The sleep/wake 

cycle (DOT/FAA/AM-95/19). Washington, DC: 

Federal Aviation Administration, Office of Aviation Medicine.

Della Rocco, P., Cruz, C., & Schreoder, D. (1995). 

Fatigue and Performance in the air traffic control 

environment. Advisory Group for Aerospace Research and Development (AGARD) conference: 

579. Neurological Limitations of Aircraft Operations: 

Human Performance Limitations (pp. SS3-1 – SS3-

9). Quebec: Canada Communication Group.

Della Rocco, P., Milburn, N., & Mertens, H. (1992). 

Comparison of performance on the Shipley Institute of 

Living Scale, air traffic control specialist selection test, 

and FAA Academy screen (DOT/FAA/AM-92/30). 

Washington, DC: Federal Aviation Administration, 

Office of Aviation Medicine.

Folkard, S. (1989, July). Shiftwork- a growing occupational hazard. Occupational Health, 182-6.

Folkard, S. (1992). Is there a “best compromise” shift 

system? Ergonomics, 35 (12), 1453-63.

Gander, P., Myhre, G., Graeber, R., Andersen, H., & 

Lauber, J. (1989). Adjustment of sleep and the 

circadian temperature rhythm after flights across 

nine time zones. Aviation, Space, and Environmental 

Medicine, 60, 733-43.

Hoddes, E., Zarcone, V., Smythe, E., Phillips, R., & 

Dement, W. (1973). Quantification of sleepiness: 

A new approach. Psychophyisology, 10 (4), 431-6.

Horne, J. & Ostberg, O. (1976). A self-assessment questionnaire to determine morningness-eveningness in 

human circadian rhythms. International Journal of 

Chronobiology, 4, 97-110.

Knauth, P. (1993). The design of shift systems. Ergonomics, 36 (1-3), 15-28.

Knauth, P. & Kiesswetter, E. (1987). A change from 

weekly to quicker shift rotations: a field study of 

discontinuous three-shift workers. Ergonomics, 30 

(9), 1311-21.
13

Lavie, P., Tzischinsky, O., Epstein, R., & Zomer, J. (1992). 

Sleep-wake cycle in shift-workers on a “clockwise” 

and “counter-clockwise” rotation system. Israel 

Journal of Medical Science, 28 (8-9), 636-44.

Luna, T., French, J., Mitcha, J., & Neville, K. (1994). 

Forward rapid rotation shiftwork in USAF air traffic 

controllers: Sleep, activity, fatigue, and mood analyses (AL/CF-TR-1994-0156). Brooks Air Force Base, 

TX: United States Air Force, Armstrong Laboratory.

Monk, T. & Folkard, S. (1985). Individual differences in 

shiftwork adjustment. In S. Folkard and T. Monk 

(Eds.) Hours of Work: Temporal Factors in Work 

Scheduling, 227-237. New York and Chichester, 

England: John Wiley and Sons Ltd.

 Rosa, R., Bonnet, M., Bootzin, R., Eastman, C., Monk, 

T., Penn, P., Tepas, D., & Walsh, J. (1990). Intervention factors for promoting adjustment to nightwork and shiftwork. In A. Scott (Ed.) Occupational 

Medicine: State of the Art Reviews, 5 (2): 391-414. 

Philadelphia, PA: Hanley & Belfus.

Tucker, P., Smith, L., Macdonald, I., & Folkard, S. (2000). 

Effects of direction of rotation in continuous and 

discontinuous 8 hour shift systems. Occupational 

and Environmental Medicine, 57 (10), 678-84.

Turek, F. (1986). Circadian principles and design of 

rotating shift work schedules. American Journal of 

Physiology, 251, R636-R638.

Watson, D., Clark, L., & Tellegen, A. (1988). Development and validation of brief measures of positive 

and negative affect: The PANAS scale. Journal of 

Personality and Social Psychology, 54 (6), 1063-70.

Wedderburn, A. (1992). How fast should the night shift 

rotate? A Rejoinder. Ergonomics, 35 (12), 1447-51.

Wilkinson, R. (1992). How fast should the night shift 

rotate? Ergonomics, 35 (12), 1425-46.

Zachary, R. ( 1986). Shipley Institute of Living Scale Revised Manual. Los Angeles: Western Psychological 

Services.

A1

Table A1

Descriptive Statistics for Self-Reported Sleep Duration from Logbooks

Shift/Sleep Period Clockwise Rotation (n=14) Counter-Clockwise Rotation (n=14)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1 5.1 1.1 4.9 1.2 5.0 0.8 4.9 1.0

Early Morning 2 5.4 0.9 5.1 1.0 5.8 0.8 6.1 0.9

Afternoon 1 7.7 1.4 7.7 1.2 8.2 1.2 7.8 1.5

Afternoon 2 7.2 1.3 7.5 1.1 7.9 0.9 7.5 1.3

Midnight 7.0 1.4 7.4 0.8 -- -- -- --

Nap before Midnight 1.1 1.7 0.6 1.1 2.2 1.6 2.2 1.4

Nap after mid 4.9 2.1 5.4 1.8 4.3 1.6 4.3 1.6

Recovery 8.3 1.7 8.3 2.1 8.6 1.7 8.4 1.4

Table A2

Descriptive Statistics for Scored Sleep Duration from Wrist Actigraphy

Shift/Sleep Period Clockwise Rotation (n=9) Counter-Clockwise Rotation (n=13)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1 5.7 1.6 5.6 1.2 5.6 0.5 5.5 0.7

Early Morning 2 6.6 0.9 6.2 2.1 6.5 0.4 6.6 0.8

Afternoon 1 7.9 1.5 7.8 0.9 8.7 1.4 8.4 1.6

Afternoon 2 7.8 1.3 7.8 1.2 8.2 0.8 7.9 1.2

Midnight 7.5 1.8 8.2 1.6 -- -- -- --

Nap before Midnight 1.7 1.8 1.3 1.4 2.8 1.6 2.9 1.3

Nap after mid 4.5 2.5 4.3 1.4 4.5 1.6 4.1 1.7

Recovery 8.3 1.8 8.5 1.7 8.9 1.6 8.9 1.0

Table A3

Descriptive Statistics for Scored Sleep Duration by Age and Rotation Condition

Shift/Sleep Period Clockwise Rotation (n=9) Counter-Clockwise Rotation (n=13)

<40 yrs old (n=4) >40 yrs old (n=5) <40 yrs old (n=7) >40 yrs old (n=6)

Week 2 mean sd mean sd mean sd mean sd

Early Morning 1 5.3 2.1 6.0 1.3 5.6 0.5 5.5 0.5

Early Morning 2 6.3 0.8 6.8 1.0 6.4 0.4 6.6 0.4

Afternoon 1 7.5 2.0 8.3 1.0 8.2 1.5 9.2 1.1

Afternoon 2 7.2 1.2 8.3 1.4 8.1 1.0 8.4 0.5

Midnight 8.7 1.1 9.7 2.0 8.7 1.6 8.9 2.3

Recovery 6.9 0.8 9.3 1.6 8.6 2.0 9.2 1.0

Week 3

Early Morning 1 5.5 1.4 5.7 1.2 5.4 0.7 5.7 0.6

Early Morning 2 5.4 1.1 6.8 2.6 6.2 0.7 7.0 0.6

Afternoon 1 7.9 1.0 7.7 0.9 7.9 1.6 9.1 1.4

Afternoon 2 7.2 0.9 8.3 1.2 7.3 1.1 8.7 0.9

Midnight 8.6 1.1 10.3 2.9 8.6 1.9 10.6 0.8

Recovery 8.4 1.5 8.6 2.1 8.9 1.0 9.0 1.2

APPENDIX A

Table of Descriptive Statistics for Study Measures
A2

Table A4

Descriptive Statistics for Sleep Onset Time

Shift/Sleep Period Clockwise Rotation (n=9) Counter-Clockwise Rotation (n=13)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1 22:32 01:47 23:01 01:22 23:13 00:22 23:10 00:37

Early Morning 2 22:03 01:03 22:32 02:11 22:16 00:36 21:59 00:55

Afternoon 1 23:51 02:01 23:51 01:59 23:26 01:04 23:04 01:00

Afternoon 2 00:06 01:05 00:11 01:01 23:34 00:39 23:42 00:43

Midnight 00:18 01:11 00:01 01:30 -- -- -- --

Nap before Midnight 15:55 02:45 15:11 02:53 15:31 01:00 15:32 01:05

Nap after mid 06:48 00:21 06:41 00:20 06:57 00:32 07:05 00:31

Recovery 23:50 02:06 00:08 01:47 23:21 01:41 23:41 01:25

Table A5

Descriptive Statistics for Awake Time

Shift/Sleep Period Clockwise Rotation (n=9) Counter-Clockwise Rotation (n=13)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1 04:24 00:32 04:41 00:24 04:48 00:25 04:46 00:26

Early Morning 2 04:39 00:22 04:42 00:22 04:42 00:28 04:36 00:30

Afternoon 1 07:59 01:32 07:47 01:34 08:13 01:26 07:33 01:27

Afternoon 2 08:00 01:37 08:02 01:34 07:53 00:45 07:41 01:08

Midnight 07:56 01:56 08:02 01:39 -- -- -- --

Nap before Midnight 18:09 02:32 17:32 02:08 18:37 01:35 18:30 01:37

Nap after mid 12:04 02:02 10:45 01:27 11:43 01:21 11:32 01:00

Recovery 08:13 01:53 08:45 01:52 08:36 01:09 08:43 00:56

Table A6

Descriptive Statistics for Sleep Quality Ratings

Shift/Sleep Period Clockwise Rotation (n=14) Counter-Clockwise Rotation (n=14)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1 12.7 4.8 12.9 4.3 12.3 3.1 12.5 2.7

Early Morning 2 14.0 3.4 13.7 3.8 13.1 3.5 14.4 3.0

Afternoon 1 14.4 2.5 14.5 3.9 14.6 2.8 15.9 2.8

Afternoon 2 14.3 3.6 14.9 2.9 15.1 2.4 13.9 3.0

Recovery 14.7 3.0 15.6 2.7 16.1 2.4 16.9 2.5
A3

Table A7

Descriptive Statistics for Stanford Sleepiness Scale Ratings

Shift Clockwise Rotation (n=14) Counter-Clockwise Rotation (n=14)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1

 Rating 1 2.9 1.1 3.3 1.4 3.0 0.7 3.4 0.9

 Rating 2 3.0 1.3 3.3 1.4 4.0 1.5 3.2 1.6

 Rating 3 2.6 1.1 3.3 1.1 3.1 1.1 3.6 1.7

 Rating 4 3.8 1.6 3.9 1.6 3.7 1.7 3.7 1.7

 Rating 5 2.7 1.1 3.4 1.3 3.0 1.3 3.1 1.4

Early Morning 2

 Rating 1 2.8 1.5 2.9 1.3 2.8 0.9 2.6 0.7

 Rating 2 3.4 1.5 3.4 1.8 3.1 0.9 3.4 1.2

 Rating 3 3.1 1.5 3.8 1.7 2.6 1.3 3.2 1.3

 Rating 4 4.2 1.3 4.1 1.8 3.6 1.2 3.7 1.5

 Rating 5 3.0 1.5 3.1 1.6 2.8 1.1 3.0 1.0

Afternoon 1

 Rating 1 1.9 1.0 2.1 1.3 2.1 0.9 1.7 0.7

 Rating 2 2.3 1.1 2.3 1.1 2.6 0.9 2.8 1.3

 Rating 3 2.4 1.3 2.2 1.4 2.8 1.1 2.6 1.4

 Rating 4 2.9 1.4 3.1 1.5 3.1 1.2 2.9 1.1

 Rating 5 2.8 1.5 2.9 1.3 2.9 1.0 2.9 1.2

Afternoon 2

 Rating 1 2.4 1.6 1.9 1.0 2.2 1.1 2.1 1.1

 Rating 2 2.9 1.7 2.8 1.2 2.9 1.7 2.4 1.3

 Rating 3 2.8 1.4 3.0 1.6 2.8 0.9 2.4 1.2

 Rating 4 3.5 1.7 3.7 1.7 3.5 1.2 3.7 1.3

 Rating 5 3.3 1.3 3.1 1.4 3.1 1.1 3.0 1.6

Midnight

 Rating 1 2.4 1.4 2.1 1.4 2.6 1.2 3.0 0.9

 Rating 2 3.4 1.5 2.9 1.4 3.0 1.2 2.8 1.1

 Rating 3 4.2 1.8 3.9 1.6 4.1 1.4 4.0 1.0

 Rating 4 4.9 1.7 5.6 1.7 6.1 1.0 5.8 1.3

 Rating 5 4.1 1.9 4.2 1.8 4.9 1.2 5.0 1.3
A4

Table A8

Descriptive Statistics for Negative Affect Ratings

Shift Clockwise Rotation (n=14) Counter-Clockwise Rotation (n=14)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1

 Rating 1 11.5 2.3 12.1 4.8 10.2 0.6 10.7 1.1

 Rating 2 11.4 2.2 11.9 4.8 10.3 0.6 10.6 0.9

 Rating 3 10.6 1.6 11.5 3.9 10.4 0.8 11.1 2.0

 Rating 4 11.1 1.8 11.7 4.5 10.2 0.6 11.6 2.8

Early Morning 2

 Rating 1 10.9 2.5 11.3 3.7 10.3 0.6 10.1 0.4

 Rating 2 12.1 3.9 11.4 3.7 10.1 0.5 10.4 0.8

 Rating 3 13.7 10.2 10.3 3.4 10.2 0.8 10.1 0.4

 Rating 4 12.2 4.1 11.1 3.7 10.1 0.3 10.4 1.1

Afternoon 1

 Rating 1 10.7 2.1 11.0 4.2 10.4 1.3 10.1 0.5

 Rating 2 10.8 2.4 12.1 4.2 10.0 0.0 10.2 0.8

 Rating 3 11.1 2.6 11.5 4.0 10.1 0.4 10.0 0.0

 Rating 4 11.2 2.6 11.4 3.7 10.5 1.3 10.1 0.3

Afternoon 2

 Rating 1 11.6 4.8 11.0 3.2 10.4 1.1 10.4 1.1

 Rating 2 11.5 4.5 10.8 2.4 10.1 0.4 10.4 0.9

 Rating 3 11.9 4.6 11.0 2.9 10.3 0.6 10.2 0.6

 Rating 4 11.7 4.3 11.2 2.8 10.1 0.4 10.6 2.1

Midnight

 Rating 1 11.2 3.4 11.2 3.2 10.2 0.4 10.1 0.4

 Rating 2 11.4 4.0 11.2 3.4 10.0 0.0 10.1 0.4

 Rating 3 11.7 4.0 11.2 2.9 10.3 0.6 10.1 0.4

 Rating 4 11.9 4.0 11.5 3.0 10.3 0.6 10.5 1.6
A5

Table A9

Descriptive Statistics for Positive Affect Ratings

Shift Clockwise Rotation (n=14) Counter-Clockwise Rotation (n=14)

Week 2 Week 3 Week 2 Week 3

mean sd mean sd mean sd mean sd

Early Morning 1

 Rating 1 22.4 8.4 20.5 8.8 18.4 6.6 16.9 7.0

 Rating 2 21.2 7.5 21.9 9.5 15.9 5.3 17.3 9.4

 Rating 3 23.1 9.6 21.0 8.8 18.4 7.2 17.1 9.8

 Rating 4 19.4 9.2 17.4 8.0 18.2 11.7 15.6 6.7

Early Morning 2

 Rating 1 22.3 8.2 20.0 6.4 19.4 6.3 18.8 6.6

 Rating 2 21.9 9.6 20.6 8.7 19.2 8.2 16.4 4.4

 Rating 3 20.8 8.2 19.9 9.5 19.2 8.7 17.9 7.5

 Rating 4 17.0 7.5 18.6 8.8 16.7 8.6 16.6 8.0

Afternoon 1

 Rating 1 26.9 7.7 25.2 10.6 22.8 6.5 23.0 9.9

 Rating 2 23.3 7.3 22.9 9.7 19.8 6.5 18.4 4.1

 Rating 3 23.6 8.1 23.9 11.6 20.8 9.3 20.4 9.0

 Rating 4 23.3 9.0 17.3 7.1 18.1 7.6 20.0 8.3

Afternoon 2

 Rating 1 25.1 9.8 26.5 10.0 22.4 9.2 20.7 9.6

 Rating 2 22.1 8.3 21.1 9.0 18.9 6.3 20.1 7.9

 Rating 3 22.0 8.7 21.8 8.9 19.4 6.4 18.9 7.9

 Rating 4 18.6 6.3 18.2 7.4 16.9 7.4 16.0 8.6

Midnight

 Rating 1 23.9 9.7 23.6 9.9 18.8 7.8 17.5 6.1

 Rating 2 18.6 6.8 19.6 6.6 19.1 9.8 18.2 8.1

 Rating 3 16.7 6.3 16.1 6.7 16.2 7.3 13.9 5.2

 Rating 4 14.7 6.3 13.1 5.1 12.5 5.6 11.9 4.4