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Background ; Previous studies have demonstrated that neurobehavioral performance fluctuates with the circadian cycle of alertness and decreases with hours of prior wake. The literature contains controversy however, on whether homeostatic and circadian pressures influence performance in an independent fashion or whether they combine interactively. Objectives: By desynchronising these two systems, this study aims to investigate the distinct influences of the circadian system and prior wakefulness on neurobehavioral performance and their manner of interaction. Methods: Eleven young healthy males lived in a time isolation laboratory for 12 consecutive days. They were placed on a 7 x 28h forced desynchrony protocol, consisting of 9.3h sleep opportunities and 18.7h wake episodes. Neurobehavioral performance was assessed during a 10- minute Psychomotor Vigilance Task (PVT). Results: PVT performance showed significant main effects of prior wake and circadian phase and an interaction effect of prior wake circadian phase. The interaction reflects the effect of prior wake on performance being greatest at the trough and upward segment of the circadian phase and least at the peal and downward segment. Alternatively, as prior wake increases the amplitude of the circadian effect on performance increases and its range drops. Conclusion: The significant interaction effect provides a strong argument that the influences of circadian and homeostatic processes on performance are interactive. The findings depict the circadian nadir as the phase point where the interaction of homeostatic and circadian pressure is greatest. Refereed/Peer-reviewed

Fatigue is a significant contributor to motor-vehicle accidents and fatalities. Shift workers are particularly susceptible to fatigue-related risks as they are often sleep-restricted and required to commute around the clock. Simple assays of performance could provide useful indications of risk in fatigue management, but their effectiveness may be influenced by changes in their sensitivity to sleep loss across the day. The aim of this study was to evaluate the sensitivity of several neurobehavioral and subjective tasks to sleep restriction (SR) at different circadian phases and their efficacy as predictors of performance during a simulated driving task. Thirty-two volunteers (M +/- SD; 22.8 +/- 2.9 years) were time-isolated for 13-days and participated in one of two 14-h forced desynchrony protocols with sleep opportunities equivalent to 8 h/24 h (control) or 4 h/24 h (SR). At regular intervals during wake periods, participants completed a simulated driving task, several neurobehavioral tasks, including the psychomotor vigilance task (PVT), and subjective ratings, including a self-assessment measure of ability to perform. Scores transformed into standardized units relative to baseline were folded into circadian phase bins based on core body temperature. Sleep dose and circadian phase effect sizes were derived via mixed models analyses. Predictors of driving were identified with regressions. Performance was most sensitive to sleep restriction around the circadian nadir. The effects of sleep restriction around the circadian nadir were larger for simulated driving and neurobehavioral tasks than for subjective ratings. Tasks did not significantly predict driving performance during the control condition or around the acrophase during the SR condition. The PVT and self-assessed ability were the best predictors of simulated driving across circadian phases during SR. These results show that simple performance measures and self-monitoring explain a large proportion of the variance in driving when fatigue-risk is high. (C) 2015 Elsevier Ltd. All rights reserved. Refereed/Peer-reviewed

The impact of sleep restriction on the endogenous circadian rhythms of hunger and satiety were examined in 28 healthy young men. Participants were scheduled to 2 ˆ 24-h days of baseline followed by 8 ˆ 28-h days of forced desynchrony during which sleep was either moderately restricted (equivalent to 6 h in bed/24 h; n = 14) or severely restricted (equivalent to 4 h in bed/24 h;n = 14). Self-reported hunger and satisfaction were assessed every 2.5 h during wake periods using visual analogue scales. Participants were served standardised meals and snacks at regular intervals and were not permitted to eat ad libitum. Core body temperature was continuously recorded with rectal thermistors to determine circadian phase. Both hunger and satiety exhibited a marked endogenous circadian rhythm. Hunger was highest, and satiety was lowest, in the biological evening(i.e., ~17:00–21:00 h) whereas hunger was lowest, and satiety was highest in the biological night(i.e., 01:00–05:00 h). The results are consistent with expectations based on previous reports and may explain in some part the decrease in appetite that is commonly reported by individuals who arerequired to work at night. Interestingly, the endogenous rhythms of hunger and satiety do not appearto be altered by severe—as compared to moderate—sleep restriction Refereed/Peer-reviewed

For the control group, there was a clear circadian rhythm in sleep propensity, with a peak of 98.5% at 5° (~05:20 h), a trough of 64.9% at 245° (~21:20 h), and an average of 82.3%. In contrast, sleep propensity for the sleep restriction group was relatively high at all circadian phases, with an average of 96.7%. For this group, the highest sleep propensity (99.0%) occurred at 60° (~09:00 h), and the lowest sleep propensity (91.3%) occurred at 265° (~22:40 h). As has been shown previously, these current data indicate that with a normal sleep-to-wake ratio, the effect of the circadian process on sleep propensity is pronounced, such that a forbidden zone for sleep exists at a phase equivalent to evening time for a normally entrained individual. However, these current data also indicate that when the ratio of sleep to wake is substantially lower than normal, this circadian effect is masked. In particular, sleep propensity is very high at all circadian phases, including those that coincide with the forbidden zone for sleep. This finding suggests that if the homeostatic pressure for sleep is sufficiently high, then the circadian drive for wakefulness can be overridden. In future studies, it will be important to determine whether or not this masking effect occurs with less severe sleep restriction, e.g., with a sleep-to-wake ratio of 1:3. Previous forced desynchrony studies have highlighted the close relationship between the circadian rhythms of core body temperature (CBT) and sleep propensity. In particular, these studies have shown that a "forbidden zone" for sleep exists on the rising limb of the CBT rhythm. In these previous studies, the length of the experimental day was either ultrashort (90 min), short (20 h), or long (28 h), and the ratio of sleep to wake was normal (i.e., 1:2). The aim of the current study was to examine the relative effects of the circadian and homeostatic processes on sleep propensity using a 28-h forced desynchrony protocol in which the ratio of sleep to wake was substantially lower than normal (i.e., 1:5). Twenty-seven healthy males lived in a time-isolation sleep laboratory for 11 consecutive days. Participants completed either a control (n = 13) or sleep restriction (n = 14) condition. In both conditions, the protocol consisted of 2 × 24-h baseline days followed by 8 × 28-h forced desynchrony days. On forced desynchrony days, the control group had 9.3 h in bed and 18.7 h of wake, and the sleep restriction group had 4.7 h in bed and 23.3 h of wake. For all participants, each 30-s epoch of time in bed was scored as sleep or wake based on standard polysomnography recordings, and was also assigned a circadian phase (360° = 24 h) based on a cosine equation fitted to continuously recorded CBT data. For each circadian phase (i.e., 72 × 5° bins), sleep propensity was calculated as the percentage of epochs spent in bed scored as sleep. Refereed/Peer-reviewed

Although a nonlinear time-of-day and prior wake interaction on performance has been well documented, two recent studies have aimed to incorporate the influences of sleep restriction into this paradigm. Through the use of sleeprestricted forced desynchrony protocols, both studies reported a time-of-day × sleep restriction interaction, as well as a time-of-day × prior wake × sleep dose three-way interaction. The current study aimed to investigate these interactions on simulated driving performance, a more complex task with ecological validity for the problem of fatigued driving. The driving performance of 41 male participants (mean ± SD: 22.8 ±2.2 yrs) was assessed on a 10- min simulated driving task with the standard deviation of lateral position (SDLAT) measured. Using a between-group design, participants were subjected to either a control condition of 9.33 h of sleep/18.66 h of wake, a moderate sleep-restriction (SR) condition of 7 h of sleep/21 h of wake, or a severe SR condition of 4.66 h of sleep/23.33 h of wake. In each condition, participants were tested at 2.5-h intervals after waking across 7 × 28-h d of forced desynchrony. Driving sessions occurred at nine doses of prior wake, within six divisions of the circadian cycle based on core body temperature (CBT). Mixed-models analyses of variance (ANOVAs) revealed significant main effects of time-of-day, prior wake, sleep debt, and sleep dose on SDLAT. Additionally, significant two-way interactions of timeof- day × prior wake and time-of-day × sleep debt, as well as significant three-way interactions of time-of-day × prior wake × sleep debt and time-of-day × sleep debt × sleep dose were observed. Although limitations such as the presence of practice effects and large standard errors are noted, the study concludes with three findings. The main effects demonstrate that extending wake, reducing sleep, and driving at poor times of day all significantly impair driving performance at an individual level. In addition to this, combining either extended wake or a sleep debt with the early morning hours greatly decreases driving performance. Finally, operating under the influence of a reduced sleep dose can greatly decrease performance at all times of the day. Refereed/Peer-reviewed

Biomathematical models are used in industry to estimate how much sleep people are likely to get on different work patterns, and how efficient and safe people are likely to be at work. Since there is evidence to suggest that individuals respond differently to sleep loss, there has been a recent focus on trying to account for individual differences. One possible approach could use past behaviour to predict future responses to similar working conditions. This study investigated the predictive value of sleep timing and duration data for a particular individual on a break between shifts relative to data from their colleagues. Sleep diaries and wrist actigraphy were collected from 306 international long-haul pilots for at least 2-weeks. Fifty layovers, equivalent in origin and destination, length and timing, were completed twice by individual pilots. Matched layovers done by other pilots (n = 2311) were also identified. Layover periods were analysed for minute-by-minute correspondence of sleep or wake (yes/no), and total sleep time (TST). Using an individual's own data improved concordance by approximately 5% relative to using a large sample of different pilots, and by 10% relative to using a random sample of 50 different pilots. Using an individual's own TST to predict their TST on an equivalent layover yielded an r value of 0.83, compared to r = 0.78 when data from a colleague was used, and r = 0.73 using different pilots in a random sample of equivalent size. The mean difference in TST using pilots’ own data was

The sleep and performance of train drivers was monitored across a 106-h rail operation between the Australian cities of Adelaide and Perth. The drivers worked alternating 8-h shift rotations across the operation and rested in specially equipped, crew-van carriages during non-work periods. The crew-van rest opportunities were associated with shorter bedtime spans, less total sleep time, and poorer sleep efficiency than sleeps initiated at home. The duration of crew-van sleeps was primarily dependent on the time of day at which the rest opportunities occurred. Overall, drivers incurred a significant cumulative sleep loss across the duration of the operation. Despite the deficit, drivers were able to sustain vigilance performance across the operation.

Human fatigue, caused by sleep loss, extended wakefulness, and/or circadian misalignment, is a major cause of workplace errors, incidents and accidents. In some industries, employees are required to undertake fitness for duty testing at the start of a shift to identify instances where their fatigue risk is elevated, so that minimisation and/or mitigation strategies can be implemented. Postural balance has been proposed as a fitness for duty test for fatigue, but it is largely untested. Therefore, the purpose of this study was to examine the impact of sleep loss, extended wakefulness and circadian phase on postural balance. Fourteen male participants spent 10 consecutive days in a sleep laboratory, including three adaptation days and eight simulated shiftwork days. To simulate a quickly rotating roster, shiftwork days were scheduled to begin 4 h later each day, and consisted of a 23.3-h wake episode and a 4.7-h sleep opportunity. Every 2.5 h during wake, balance was measured while standing as still as possible on a force platform with eyes open for one minute, and eyes closed for one minute. Subjective sleepiness was assessed using the Karolinska Sleepiness Scale. Core body temperature, continuously recorded with rectal thermistors, was used to determine circadian phase. For measures of postural balance and subjective sleepiness, data were analysed using three separate repeated measures ANOVA with two within-subjects factors: circadian phase (six phases) and prior wake (nine levels). For subjective sleepiness, there was a significant effect of prior wake and circadian phase. In particular, sleepiness increased as prior wake increased, and was higher during biological night-time than biological daytime. For the eyes open balance task, there was no effect of prior wake or circadian phase. For the eyes closed balance task, there was a significant effect of circadian phase such that balance was poorer during the biological night-time than biological daytime, there was no effect of prior wake. These results indicate that postural balance may be a viable tool for assessing fatigue associated with time of day, but may not be useful for assessing fatigue associated with extended hours of wake. Refereed/Peer-reviewed

Subjective ratings of fatigue are increasingly being used as part of a suite of tools to assess fatigue-related risk on the road and in the workplace. There is some debate however, as to whether individuals can accurately gauge their own fatigue states, particularly under conditions of sleep restriction. It is also unclear which references are used by individuals to assess fatigue – for example prior sleep, time of day, workload, or previous ratings. The current study used a sophisticated laboratory protocol to examine the independent contributions of sleep, circadian phase and sleep debt to fatigue ratings. Importantly, participants had no knowledge of time of day, how much sleep they were getting, or how long they were awake. Twenty-eight healthy, young males participated in one of two conditions of a 28 h forced desynchrony protocol – severe sleep restriction (4.7 h sleep and 23.3 h wake) or moderate sleep restriction (7 h sleep and 21 h wake). Fatigue ratings were provided prior to and following each sleep period using the Samn–Perelli fatigue scale. Repeated measures ANOVAs were used to analyse the effects of circadian phase, sleep dose and study day. Results demonstrated an effect of circadian phase on both pre-sleep and post-sleep fatigue ratings. The significant effect of study day is interpreted as an effect of circadian time, as opposed to accumulating sleep debt. An effect of sleep dose was only seen in post-sleep fatigue ratings. The findings suggest that post-sleep fatigue ratings may be sensitive to prior sleep and may be useful as an indicator of fatigue-related risk, particularly when triangulated with information about recent total sleep time. Refereed/Peer-reviewed

Conclusions: The performance consequence of sleep restriction in our study was prominent during the biological night, even when the prior wake duration was short, and this performance consequence was in forms of waking state instability. This result is likely due to acute homeostatic sleep pressure remaining high despite the sleep episode. Study Objectives: The homeostatic-circadian regulation of neurobehavioral functioning is not well understood in that the role of sleep dose in relation to prior wake and circadian phase remains largely unexplored. The aim of the present study was to examine the neurobehavioral impact of sleep dose at different combinations of prior wake and circadian phase. Design: A between-participant design involving 2 forced desynchrony protocols varying in sleep dose. Both protocols comprised 7 repetitions of a 28-h sleep/wake cycle. The sleep dose in a standard protocol was 9.33 h per 28-h day and 4.67 h in a sleep-restricted protocol. Setting: A time-isolation laboratory at the Centre for Sleep Research, the University of South Australia. Participants: A total of 27 young healthy males participated in the study with 13 in the standard protocol (age 22.5+- 2.2 y) and 14 in the sleep-restricted protocol (age 21.8 +-3.8 y). Interventions: Wake periods during both protocols were approximately 4 h delayed each 28-h day relative to the circadian system, allowing performance testing at different combinations of prior wake and circadian phase. The manipulation in sleep dose between the 2 protocols, therefore, allowed the impact of sleep dose on neurobehavioral performance to be examined at various combinations of prior wake and circadian phase. Measurements and Results: Neurobehavioral function was assessed using the psychomotor vigilance task (PVT). There was a sleep dose x circadian phase interaction effect on PVT performance such that sleep restriction resulted in slower and more variable response times, predominantly during the biological night. This interaction was not altered by prior wakefulness, as indicated by a nonsignificant sleep dose x circadian phase x prior wake interaction. Refereed/Peer-reviewed

Most of the research related to human fatigue in the aviation industry has focussed on long-haul pilots, but short-haul pilots also experience elevated levels of fatigue. The aim of this study was to examine the impact of early start times on the amount of sleep obtained prior to duty and on fatigue levels at the start of duty. Seventy short-haul pilots collected data regarding their duty schedule and sleep/wake behaviour for at least two weeks. Data were collected using self-report duty/sleep diaries and wrist activity monitors. Mixed-effects regression analyses were used to examine the effects of duty start time (04:00–10:00 h) on (i) the total amount of sleep obtained in the 12 h prior to the start of duty and (ii) self-rated fatigue level at the start of duty. Both analyses indicated significant main effects of duty start time. In particular, the amount of sleep obtained in the 12 h prior to duty was lowest for duty periods that commenced between 04:00 and 05:00 h (i.e. 5.4 h), and greatest for duty periods that commenced between 09:00 and 10:00 h (i.e. 6.6 h). These data indicate that approximately 15 min of sleep is lost for every hour that the start of duty is advanced prior to 09:00 h. In addition, self-rated fatigue at the start of duty was highest for duty periods that commenced between 04:00 and 05:00 h, and lowest for duty periods that commenced between 09:00 and 10:00 h. Airlines should implement a fatigue risk management system (FRMS) for short-haul pilots required to work early-morning shifts. One component of the FRMS should be focussed on the production of ‘fatigue-friendly’ rosters. A second component of the FRMS should be focussed on training pilots to optimise sleep opportunities, to identify circumstances where the likelihood of fatigue is elevated, and to manage the risks associated with fatigue-related impairment. Refereed/Peer-reviewed