Performance, sleep and circadian phase during a week of simulated night work

The current study investigated changes in night-time performance, daytime sleep, and circadian phase during a week of simulated shift work. Fifteen young subjects participated in an adaptation and baseline night sleep, directly followed by seven night shifts. Subjects slept from approximately 0800 hr until they naturally awoke. Polysomnographic data was collected for each sleep period. Saliva samples were collected at half hourly intervals, from 2000 hr to bedtime. Each night, performance was tested at hourly intervals. Analysis indicated that there was a significant increase in mean performance across the week. In general, sleep was not negatively affected. Rather, sleep quality appeared to improve across the week. However, total sleep time (TST) for each day sleep was slightly reduced from baseline, resulting in a small cumulative sleep debt of 3.53 (SD = 5.62) hours. Finally, the melatonin profile shifted across the week, resulting in a mean phase delay of 5.5 hours. These findings indicate that when sleep loss is minimized and a circadian phase shift occurs, adaptation of performance can occur during several consecutive night shifts.     

Bright-light exposure during daytime sleeping affects nocturnal melatonin secretion after simulated night work

The guidelines for night and shift workers recommend that after night work, they should sleep in a dark environment during the daytime. However, staying in a dark environment during the daytime reduces nocturnal melatonin secretion and delays its onset. Daytime bright-light exposure after night work is important for melatonin synthesis the subsequent night and for maintaining the circadian rhythms. However, it is not clear whether daytime sleeping after night work should be in a dim- or a bright-light environment for maintaining melatonin secretion. The aim of this study, therefore, was to evaluate the effect of bright-light exposure during daytime sleeping on nocturnal melatonin secretion after simulated night work. Twelve healthy male subjects, aged 24.8 ± 4.6 (mean ± SD), participated in 3-day sessions under two experimental conditions, bright light or dim light, in a random order. On the first day, the subjects entered the experimental room at 16:00 and saliva samples were collected every hour between 18:00 and 00:00 under dim-light conditions. Between 00:00 and 08:00, they participated in tasks that simulated night work. At 10:00 the next morning, they slept for 6 hours under either a bright-light condition (>3000 lx) or a dim-light condition (<50 lx). In the evening, saliva samples were collected as on the first day. The saliva samples were analyzed for melatonin concentration. Activity and sleep times were recorded by a wrist device worn throughout the experiment. In the statistical analysis, the time courses of melatonin concentration were compared between the two conditions by three-way repeated measurements ANOVA (light condition, day and time of day). The change in dim light melatonin onset (ΔDLMO) between the first and second days, and daytime and nocturnal sleep parameters after the simulated night work were compared between the light conditions using paired t-tests. The ANOVA results indicated a significant interaction (light condition and3 day) (p = .006). Post hoc tests indicated that in the dim-light condition, the melatonin concentration was significantly lower on the second day than on the first day (p = .046); however, in the bright-light condition, there was no significant difference in the melatonin concentration between the days (p = .560). There was a significant difference in ΔDLMO between the conditions (p = .015): DLMO after sleeping was advanced by 11.1 ± 17.4 min under bright-light conditions but delayed for 7.2 ± 13.6 min after sleeping under dim-light conditions. No significant differences were found in any sleep parameter. Our study demonstrated that daytime sleeping under bright-light conditions after night work could not reduce late evening melatonin secretion until midnight or delay the phase of melatonin secretion without decreasing the quality of the daytime sleeping. Thus, these results suggested that, to enhance melatonin secretion and to maintain their conventional sleep–wake cycle, after night work, shift workers should sleep during the daytime under bright-light conditions rather than dim-light conditions.     

Train drivers' sleep quality and quantity during extended relay operations

Relay operations are an important mode of freight transportation within Australia. Relay work requires multiple crews to drive the train continuously from one specified destination to another and return. Importantly, the nature of relay work requires train drivers to sleep on-board during designated resting shifts. The main aim of the present study was to investigate the quality and quantity of sleep obtained in on-board rest facilities (relay vans) during extended (four-day) relay operations. Drivers (n=9) working the Port Augusta to Darwin relay operation volunteered to participate. The first leg of the trip typically took 40 h followed by an overnight stay in Darwin (between 8-12 h) prior to return. Two crews, each consisting of two drivers, changed every 8 h, giving the crew an 8 h rest in the relay van prior to each 8 h working shift. Using polysomnography, home sleep data were collected prior to and following each trip using a standard five-channel EEG montage. All sleep periods during the relay trip (including Darwin) were also recorded. Additionally, subjective sleep quality ratings were recorded following each sleep period. Analyses revealed that the quantity of sleep obtained in the relay vans (3.3 h) was significantly reduced compared to home (6.8 h). In general, the total sleep time was increased at night and reduced during the day. In terms of quality, sleep onset latency, sleep efficiency, and amount of slow wave and rapid eye movement sleep did not differ significantly between home and the relay vans. The results of the study highlight sleep quantity as the main concern during extended relay operations. Future research should focus on investigating the subjective and objective impact of this sleep reduction on waking functions.     

Postprandial metabolic profiles following meals and snacks eaten during simulated night and day shift work

Shift workers are known to have an increased risk of developing cardiovascular disease (CVD) compared with day workers. An important factor contributing to this increased risk could be the increased incidence of postprandial metabolic risk factors for CVD among shift workers, as a consequence of the maladaptation of endogenous circadian rhythms to abrupt changes in shift times. We have previously shown that both simulated and real shift workers showed relatively impaired glucose and lipid tolerance if a single test meal was consumed between 00:00-02:00 h (night shift) compared with 12:00-14:00 h (day shift). The objective of the present study was to extend these observations to compare the cumulative metabolic effect of consecutive snacks/meals, as might normally be consumed throughout a period of night or day shift work. In a randomized crossover study, eight healthy nonobese men (20-33 yrs, BMI 20-25kg/m2) consumed a combination of two meals and a snack on two occasions following a standardized prestudy meal, simulating night and day shift working (total energy 2500 kcal: 40% fat, 50% carbohydrate, 10% protein). Meals were consumed at 01:00/ 13:00 h and 07:00/19:00h, and the snack at 04:00/16:00 h. Blood was taken after an overnight fast, and for 8 h following the first meal on each occasion, for the measurement of glucose, insulin, triacylglycerol (TAG), and nonesterified fatty acids (NEFA). RM-ANOVA (factors time and shift) showed a significant effect of shift for plasma TAG, with higher levels on simulated night compared to day shift (p < 0.05). There was a trend toward an effect of shift for plasma glucose, with higher plasma glucose at night (p = 0.08), and there was a time-shift interaction for plasma insulin levels (p < 0.01). NEFA levels were unaffected by shift. Inspection of the area under the plasma response curve (AUC) following each meal and snack revealed that the differences in lipid tolerance occurred throughout the study, with greatest differences occurring following the mid-shift snack. In contrast, glucose tolerance was relatively impaired following the first night-time meal, with no differences observed following the second meal. Plasma insulin levels were significantly lower following the first meal (p < 0.05), but significantly higher following the second meal (p < 0.01) on the simulated night shift. These findings confirm our previous observations of raised postprandial TAG and glucose at night, and show that sequential meal ingestion has a more pronounced effect on subsequent lipid than carbohydrate tolerance.     

Effectiveness of a red-visor cap for preventing light-induced melatonin suppression during simulated night work

Bright light at night improves the alertness of night workers. Melatonin suppression induced by light at night is, however, reported to be a possible risk factor for breast cancer. Short-wavelength light has a strong impact on melatonin suppression. A red-visor cap can cut the short-wavelength light from the upper visual field selectively with no adverse effects on visibility. The purpose of this study was to investigate the effects of a red-visor cap on light-induced melatonin suppression, performance, and sleepiness at night. Eleven healthy young male adults (mean age: 21.2±0.9 yr) volunteered to participate in this study. On the first day, the subjects spent time in dim light (<15 lx) from 20:00 to 03:00 to measure baseline data of nocturnal salivary melatonin concentration. On the second day, the subjects were exposed to light for four hours from 23:00 to 03:00 with a nonvisor cap (500 lx), red-visor cap (approx. 160 lx) and blue-visor cap (approx. 160 lx). Subjective sleepiness and performance of a psychomotor vigilance task (PVT) were also measured on the second day. Compared to salivary melatonin concentration under dim light, the decrease in melatonin concentration was significant in a nonvisor cap condition but was not significant in a red-visor cap condition. The percentages of melatonin suppression in the nonvisor cap and red-visor cap conditions at 4 hours after exposure to light were 52.6±22.4% and 7.7±3.3%, respectively. The red-visor cap had no adverse effect on performance of the PVT, brightness and visual comfort, though it tended to increase subjective sleepiness. These results suggest that a red-visor cap is effective in preventing melatonin suppression with no adverse effects on vigilance performance, brightness and visibility.     

An endogenous circadian rhythm in sleep inertia results in greatest cognitive impairment upon awakening during the biological night

Sleep inertia is the impaired cognitive performance immediately upon awakening, which decays over tens of minutes. This phenomenon has relevance to people who need to make important decisions soon after awakening, such as on-call emergency workers. Such awakenings can occur at varied times of day or night, so the objective of the study was to determine whether or not the magnitude of sleep inertia varies according to the phase of the endogenous circadian cycle. Twelve adults (mean, 24 years; 7 men) with no medical disorders other than mild asthma were studied. Following 2 baseline days and nights, subjects underwent a forced desynchrony protocol composed of seven 28-h sleep/wake cycles, while maintaining a sleep/wakefulness ratio of 1:2 throughout. Subjects were awakened by a standardized auditory stimulus 3 times each sleep period for sleep inertia assessments. The magnitude of sleep inertia was quantified as the change in cognitive performance (number of correct additions in a 2-min serial addition test) across the first 20 min of wakefulness. Circadian phase was estimated from core body temperature (fitted temperature minimum assigned 0 degrees ). Data were segregated according to: (1) circadian phase (60 degrees bins); (2) sleep stage; and (3) 3rd of the night after which awakenings occurred (i.e., tertiary 1, 2, or 3). To control for any effect of sleep stage, the circadian rhythm of sleep inertia was initially assessed following awakenings from Stage 2 (62% of awakening occurred from this stage; n = 110). This revealed a significant circadian rhythm in the sleep inertia of cognitive performance (p = 0.007), which was 3.6 times larger during the biological night (circadian bin 300 degrees , approximately 2300-0300 h in these subjects) than during the biological day (bin 180 degrees , approximately 1500-1900 h). The circadian rhythm in sleep inertia was still present when awakenings from all sleep stages were included (p = 0.004), and this rhythm could not be explained by changes in underlying sleep drive prior to awakening (changes in sleep efficiency across circadian phase or across the tertiaries), or by the proportion of the varied sleep stages prior to awakenings. This robust endogenous circadian rhythm in sleep inertia may have important implications for people who need to be alert soon after awakening.     

The impact of sleep timing and bright light exposure on attentional impairment during night work

The prevalence of hazardous incidents induced by attentional impairment during night work and ensuing commute times is attributable to circadian misalignment and increased sleep pressure. In a 10-day shift work simulation protocol (4 day shifts and 3 night shifts), the efficacies of 2 countermeasures against nighttime (2300 to 0700 h) attentional impairment were compared: (1) Morning Sleep (0800 to 1600 h; n = 18) in conjunction with a phase-delaying light exposure (2300 to 0300 h), and (2) Evening Sleep (1400 to 2200 h; n = 17) in conjunction with a phase-advancing light exposure (0300 to 0700 h). Analysis of the dim light salivary melatonin onset indicated a modest but significant circadian realignment in both sleep groups (evening sleep: 2.27 +/- 0.6 h phase advance, p < 0.01; morning sleep: 4.98 +/- 0.43 h phase delay, p < 0.01). Daytime sleep efficiency and total sleep time did not differ between them or from their respective baseline sleep (2200 to 0600 h; p > 0.05). However, on the final night shift, the evening sleep subjects had 37% fewer episodes of attentional impairment (long response times: 22 +/- 4 vs. 35 +/- 4; p = 0.02) and quicker responses (p < 0.01) on the Psychomotor Vigilance Task than their morning sleep counterparts. Their response speed recovered to near daytime levels (p = 0.47), whereas those of the morning sleep subjects continued to be slower than their daytime levels (p = 0.008). It is concluded that partial circadian realignment to night work in combination with reduced homeostatic pressure contributed to the greater efficacy of a schedule of Evening Sleep with a phase-advancing light exposure as a countermeasure against attentional impairment, over a schedule of Morning Sleep with a phase-delaying light exposure. These results have important implications for managing patients with shift work disorder.     

Daytime napping after a night of sleep loss decreases sleepiness, improves performance, and causes beneficial changes in cortisol and interleukin-6 secretion

Sleep loss has been associated with increased sleepiness, decreased performance, elevations in inflammatory cytokines, and insulin resistance. Daytime napping has been promoted as a countermeasure to sleep loss. To assess the effects of a 2-h midafternoon nap following a night of sleep loss on postnap sleepiness, performance, cortisol, and IL-6, 41 young healthy individuals (20 men, 21 women) participated in a 7-day sleep deprivation experiment (4 consecutive nights followed by a night of sleep loss and 2 recovery nights). One-half of the subjects were randomly assigned to take a midafternoon nap (1400-1600) the day following the night of total sleep loss. Serial 24-h blood sampling, multiple sleep latency test (MSLT), subjective levels of sleepiness, and psychomotor vigilance task (PVT) were completed on the fourth (predeprivation) and sixth days (postdeprivation). During the nap, subjects had a significant drop in cortisol and IL-6 levels (P < 0.05). After the nap they experienced significantly less sleepiness (MSLT and subjective, P < 0.05) and a smaller improvement on the PVT (P < 0.1). At that time, they had a significant transient increase in their cortisol levels (P < 0.05). In contrast, the levels of IL-6 tended to remain decreased for approximately 8 h (P = 0.1). We conclude that a 2-h midafternoon nap improves alertness, and to a lesser degree performance, and reverses the effects of one night of sleep loss on cortisol and IL-6. The redistribution of cortisol secretion and the prolonged suppression of IL-6 secretion are beneficial, as they improve alertness and performance.     

A week of simulated night work delays salivary melatonin onset

In most studies, the magnitude and rate of adaptation to various night work schedules is assessed using core body temperature as the marker of circadian phase. The aim of the current study was to assess adaptation to a simulated night work schedule using salivary dim light melatonin onset (DLMO) as an alternative circadian phase marker. It was hypothesised that the night work schedule would result in a phase delay, manifest in relatively later DLMO, but that this delay would be somewhat inhibited by exposure to natural light. Participants worked seven consecutive simulated 8-hour night shifts (23:00-07:00 h). By night 7, there was a mean cumulative phase delay of 5.5 hours, equivalent to an average delay of 0.8 hours per day. This indicates that partial circadian adaptation occurred in response to the simulated night work schedule. The radioimmunoassay used in the current study provides a sensitive assessment of melatonin concentration in saliva that can be used to determine DLMO, and thus provides an alternative phase marker to core body temperature, at least in laboratory studies.     

Morning melatonin has limited benefit as a soporific for daytime sleep after night work

Exogenous melatonin administration in humans is known to exert both chronobiotic (phase shifting) and soporific effects. In a previous study in our lab, young, healthy, subjects worked five consecutive simulated night shifts (23:00 to 07:00 h) and slept during the day (08:30 to 15:30 h). Large phase delays of various magnitudes were produced by the study interventions, which included bright light exposure during the night shifts, as assessed by the dim light melatonin onset (DLMO) before (baseline) and after (final) the five night shifts. Subjects also ingested either 1.8 mg sustained-release melatonin or placebo before daytime sleep. Although melatonin at this time should delay the circadian clock, this previous study found that it did not increase the magnitude of phase delays. To determine whether melatonin had a soporific effect, we controlled the various magnitudes of phase delay produced by the other study interventions. Melatonin (n=18) and placebo (n=18) groups were formed by matching a melatonin participant with a placebo participant that had a similar baseline and final DLMO (±1 h). Sleep log measurements of total sleep time (TST) and actigraphic measurements of sleep latency, TST, and three movement indices for the two groups were examined. Although melatonin was associated with small improvements in sleep quality and quantity, the differences were not statistically significant by analysis of variance. However, binomial analysis indicated that melatonin participants were more likely to sleep better than their placebo counterparts on some days with some measures. It was concluded that, the soporific effect of melatonin is small when administered prior to 7 h daytime sleep periods following night shift work.     

Heart rate variability in healthy humans during night sleep

Polysomnography has been performed and heart rate variability has been studied during night sleep in healthy young subjects of both sexes. It has been shown that, on average, a shift in autonomic support towards parasympathicotonia occurs during sleep, with a maximum at stages III and IV of NREM sleep. At stage II of NREM sleep and during REM sleep, a short-term activation of the sympathetic nervous system comparable to wakefulness is observed.     

Time-of-day mediates the influences of extended wake and sleep restriction on simulated driving

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 sleep-restricted 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 time-of-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.     

The effects of extended nap periods on cognitive,physiological and subjective responses under simulated night shift conditions

Extended nap opportunities have been effective in maintaining alertness in the context of extended night shifts (+12?h). However, there is limited evidence of their efficacy during 8-h shifts. Thus, this study explored the effects of extended naps on cognitive, physiological and perceptual responses during four simulated, 8-h night shifts. In a laboratory setting, 32 participants were allocated to one of three conditions. All participants completed four consecutive, 8-h night shifts, with the arrangements differing by condition. The fixed night condition worked from 22h00 to 06h00, while the nap early group worked from 20h00 to 08h00 and napped between 00h00 and 03h20. The nap late group worked from 00h00 to 12h00 and napped between 04h00 and 07h20. Nap length was limited to 3 hours and 20 minutes. Participants performed a simple beading task during each shift, while also completing six to eight test batteries roughly every 2?h. During each shift, six test batteries were completed, in which the following measures were taken. Performance indicators included beading output, eye accommodation time, choice reaction time, visual vigilance, simple reaction time, processing speed and object recognition, working memory, motor response time and tracking performance. Physiological measures included heart rate and tympanic temperature, whereas subjective sleepiness and reported sleep length and quality while outside the laboratory constituted the self reported measures. Both naps reduced subjective sleepiness but did not alter the circadian and homeostatic-related changes in cognitive and physiological measures, relative to the fixed night condition. Additionally, there was evidence of sleep inertia following each nap, which resulted in transient reductions in certain perceptual cognitive performance measures. The present study suggested that there were some benefits associated with including an extended nap during 8-h night shifts. However, the effects of sleep inertia need to be effectively managed to ensure that post-nap alertness and performance is maintained.     

Computer simulated discharges of thalamic ventrobasal neurones during sleep and wakefulness

A theoretical model is described which in response to combinations of poissonian pulse trains with different mean frequencies on three independent incoming lines, generated output signals simulating spontaneous discharges of thalamic ventrobasal (VB) neurones during sleep and wakefulness. Some dynamic neuronal properties as refractoriness, facilitation, short term memory were simulated and characteristics of response to single pulses on different lines properly selected to reproduce those exhibited by VB neurones upon artificial stimulation of thalamic afferent systems. The data obtained from the model are briefly discussed in relation to possible contributions of specific and nonspecific afferent systems in producing spontaneous VB discharges characteristic of different levels of vigilance.