Shift work influences multiple sclerosis risk.

BACKGROUND: An association between working shift at a young age and subsequent risk for multiple sclerosis (MS) has been observed. OBJECTIVE: To investigate whether this finding could be replicated, and to further explore the influence of age at first exposure to shift work. METHODS: Using a Swedish population-based, case-control study (2337 cases and 4904 controls), the incidence of MS among subjects whom had worked shifts was compared with that of those whom had not, by calculating odds ratios (ORs) with 95% confidence intervals (CIs) by means of logistic regression. RESULTS: The OR of developing MS was 1.5 (95% CI 1.2-1.8) among those whom started working shifts before age 20, whereas a less pronounced association was observed among those whom started working shifts at age 20 or later (OR 1.2; 95% CI 1.1-1.4). The effect of shift work was more pronounced among subjects whom had been exposed at a young age, regardless of the duration between the start of shift work and disease onset. CONCLUSION: Some aspects of adolescence seem to be of great importance, regarding the impact of shift work on MS risk. Circadian disruption and sleep deprivation may contribute towards explaining the association; however, the exact mechanisms behind our observations remain to be elucidated.

The impact of free-time activities on sleep, recovery and well-being.

We compared the effects of different types of free-time activity on subjective and objective indices of sleep, recovery and well-being in an experimental field study. Twelve participants spent four consecutive evenings after work in each of three conditions: pursuing quiet leisure activities at home; undertaking active leisure pursuits; doing additional work. Ratings of rest and recuperation, and of satisfaction, were lowest in the additional work condition. There were few other differences between conditions. However, being satisfied with one's evening activities (regardless of which experimental condition was being undertaken) was associated with improved subsequent sleep (self-reported). Evening activities involving lower mental effort were also associated with better-rated sleep, as well as improved recuperation and fatigue the next day. It is concluded that the nature of activity per se may be less important than (1) whether the activity accords with individual preference and (2) the cumulative demands of daytime and evening activities.

Beyond the three-process model of alertness: estimating phase, time on shift, and successive night effects.

This paper starts by summarizing the development and refinement of the additive three-process model of alertness first published by Folkard and Akerstedt in 1987. It reviews some of the successes that have been achieved by the model in not only predicting variations in subjective alertness on abnormal sleep-wake schedules but also in accounting for objective measures of sleep latency and duration. Nevertheless, predictions derived from the model concerning alertness on different shifts, and over successive night shifts, are difficult to reconcile with published data on accident risk. In light of this, we have examined two large sets of alertness ratings with a view to further refining the model and identifying additional factors that may influence alertness at any given point in time. Our results indicate that, at least for the range of sleep durations and wake-up times commonly found on rotating shift systems, we may assume the phase of the endogenous circadian component of alertness (process C) to be "set" by the time of waking. Such an assumption considerably enhanced the predictive power of the model and yielded remarkably similar phase estimates to those obtained by maximizing the post-hoc fit of the model. We then examined the manner in which obtained ratings differed from predicted values over a complete 8-day cycle of two, 12-h shift systems. This revealed a pronounced "first night compensation effect" that resulted in shift workers rating themselves as progressively more alert than would be predicted over the course of the first night shift. However, this appeared to be achieved only at the cost of lowered ratings on the second night shift. Finally, we were able to identify a "time on shift" effect whereby, with the exception of the first night shift, alertness ratings decreased over the course of each shift before showing a modest "end effect." We conclude that the identification of these additional components offers the possibility that in the future we may be able to predict trends in accident risk on abnormal sleep-wake schedules.

The impact of work time control on physicians' sleep and well-being.

Physicians' work schedules are an important determinant of their own wellbeing and that of their patients. This study considers whether allowing physicians control over their work hours ameliorates the effects of demanding work schedules. A questionnaire was completed by hospital physicians regarding their work hours (exposure to long shifts, short inter-shift intervals, weekend duties, night duties, unpaid overtime; and work time control), sleep (quantity and disturbance) and wellbeing (burnout, stress and fatigue). Work time control moderated the negative impact that frequent night working had upon sleep quantity and sleep disturbance. For participants who never worked long shifts, work time control was associated with fewer short sleeps, but this was not the case for those who did work long shifts. Optimizing the balance between schedule flexibility and patient needs could enhance physicians' sleep when working the night shift, thereby reducing their levels of fatigue and enhancing patient care.

The circadian covariation of fatigue and urinary melatonin.

Twelve healthy male subjects were kept under constant conditions (no sleep, isolation from time cues, controlled activity, etc.) for 64 hr. Urinary melatonin values, self-rated sleepiness, and vigilance performance scores were obtained every 3 hr. All variables showed a pronounced circadian rhythmicity. Vigilance performance and self-rated sleepiness showed, in addition, a gradual decrease and increase, respectively, with increasing sleep deprivation. The correlation of melatonin with performance and ratings was highly significant, high melatonin levels being associated with reduced performance and increased sleepiness. Aligning self-ratings and behavioral data with respect to the melatonin troughs and peaks showed that the former coincided with performance and alertness peaks and the latter with the troughs. It was concluded that under these conditions there is a strong circadian covariation between melatonin and indices of fatigue/sleepiness.

Sleepiness as a consequence of shift work.

The results reviewed clearly demonstrate that shift work is associated with increased subjective, behavioral, and physiological sleepiness. Apparently, the effects are due to combined circadian and homeostatic (sleep loss) influences. Sleepiness is particularly pronounced during the night shift, and may terminate in actual incidents of falling asleep at work. In some occupations this clearly constitutes a hazard that may endanger human lives and have large economic consequences. These risks clearly involve a larger number of people and should be of great significance to society.

Validation of the S and C components of the three-process model of alertness regulation.

This paper summarizes work to validate and develop further the homeostatic and circadian component of a quantitative (computerized) three-process model for predicting alertness/sleepiness in daily living. The model uses sleep data as input and contains circadian and homeostatic components (amount of prior wake and amount of prior sleep), which are summed to yield predicted alertness on a scale between 1 and 16. The present validation was carried out using regression analysis, with sleepiness-related electroencephalographic parameters (alpha power density) from field and laboratory studies as criteria. The results showed that variations in alpha-power density in truck drivers, train drivers and laboratory subjects could be predicted with considerable accuracy (r2 > 0.70) from the model, as could subjective alertness. Levels < or = 7 on the 16-point scale were defined as critically low alertness. The paper also describes a simplified, graphic, paper version of the computation model, visualized as a two-dimensional "alertness nomogram". It is suggested that the studied components of the model may serve as tools for evaluating work/rest schedules in terms of sleep-related safety risks.

The dynamics of the first sleep cycle.

Eight subjects participated in an experiment in which sleep stages and electroencephalographic (EEG) power density during the first sleep cycles (and where such appeared, also second cycles) were studied in a design involving 8, 4, 2 or 0 hr of progressively postponed night-time sleep. Each of these four manipulations was followed by a day-time sleep beginning at 1100 hr. No significant changes in the duration of the first sleep cycle appeared. As the prior sleep loss increased both SWE (slow-wave energy; accumulated EEG delta power density) and SWA (slow-wave activity; EEG delta power per minute) increased during the 1100-hr sleeps. This was observed for the entire cycles, the nonrapid eye movement (NREM) periods, and the SWS periods, respectively. SWS latency decreased and SWS duration increased, respectively, markedly with prior waking. Also, for the progressively postponed sleeps (started at 2300 hr, 0300 hr, 0500 hr and 1100 hr) there were changes, but not as clear. After 28 hr of continuous waking there was a marked increase of SWA during SWS. Also, at this level there was a spill over of SWA to the second cycle. It is suggested that there might be a limit to the amount and intensity of SWS that can be accommodated in the first sleep cycle and that this limit is reached before the appearance of REM sleep.

Stability of day and night sleep--a two-year follow-up of EEG parameters in three-shift workers.

Twenty rotating three-shift workers participated. Night and day sleep in connection with work on afternoon and night shifts, respectively, were recorded using 24-hour polysomnographic recording techniques. The procedure was repeated 2 years later. Both day and night sleep showed high significant correlations between years for rapid eye movement (REM) sleep, slow wave sleep (SWS-stages 3 + 4), total sleep time, slow wave energy, in the delta band (obtained via spectral analysis) and subjective sleep quality. Stage 2, stage 1, percent waking, sleep latency, SWS latency and REM latency were not correlated across years. None of the variables showed a significant difference between years. It was concluded that core variables of sleep show considerable interindividual stability across time and that a 2-year exposure to rotating shift work does not affect sleep in experienced shift workers.

Disturbed sleep while being on-call: an EEG study of ships' engineers.

In order to investigate the effects of on-call duty on sleep and wakefulness, five male ships' engineers were studied using electroencephalogram (EEG) and electrocardiogram (ECG) recordings and subjective ratings. Sleep during on-call nights (two alarms) was shortened and contained less slow wave sleep (SWS) and rapid eye movement (REM) sleep, lower spectral power density, and a higher heart rate. Many of the effects were observable before any alarms had occurred. Rated sleep quality was lower, and sleepiness was higher during the subsequent day. It was suggested that the effects were due to apprehension/uneasiness induced by the prospect of being awakened by an alarm.

Sleep in a truck berth.

The aim of the present pilot study was to study the effects of sleep in a truck berth. Experiment A included eight subjects who slept during two conditions (laboratory and in a truck berth during quiet conditions). Experiment B included two conditions (truck-berth sleep during quiet and noisy/disturbed conditions, respectively); six subjects participated. Polysomnography was recorded and ratings of sleep quality and postsleep sleepiness were made. During the truck-berth conditions, noise was continuously recorded. When two-tailed t tests were used, the results showed no significant effects (alpha level = 0.05) for any of the experiments. However, when one-tailed tests were used, experiment A showed a longer rapid eye movement (REM) latency for the truck-berth condition. Experiment B showed less-refreshing sleep for the disturbed condition (one-tailed test). The noise level was significantly higher during the disturbed condition. The results showed that electroencephalograph (EEG)-recorded sleep was not affected by sleeping in a truck, even when the truck was parked at a noisy location (truck terminal). However, considering some limitations of the experiments, for example small sample size, lack of adaptation night, etc., the present results should be interpreted with some caution and need to be replicated.

Body temperature and sleep at different times of day.

Six healthy male subjects were exposed to seven different bedtime conditions, one per week. Bedtimes were scheduled in 4-h intervals, resulting in times without sleep ranging from 16 h to 40 h. The influences of overt zeitgebers were minimized and the subjects were allowed to sleep until they felt they had slept enough. Rectal temperature was measured continuously and showed a circadian rhythm during both sleeping and waking. A fall in temperature immediately after sleep onset was noted at all bedtimes except at 0700 and 1900 h. In the majority of cases temperature rose toward the end of sleep, i.e., awakenings tended to occur during the rising phase of the circadian temperature rhythm. It was suggested that the phase of the body temperature rhythm is an important factor in determining sleep duration, mainly through terminating sleep during the high or rising parts of the rhythm.

The circadian variation of experimentally displaced sleep.

In a group of 6 male subjects sleep was displaced to seven different times of day (one displacement condition per week). The subjects were isolated from external time cues (daylight, clocks, noise) and sleep was allowed to terminate spontaneously. The results showed a pronounced time-of-day variation of total sleep time, stage 2, and rapid eye movement (REM) sleep. Maxima occurred after bedtimes at 1900 hr and 2300 hr, while the minima occurred after bedtimes at 0700 hr and 1100 hr. The latter also was the time of maximum propensity to wake up. Slow wave sleep showed a rapid decrease from high initial levels, irrespective of time of day. Ratings of sleepiness showed a highly significant circadian variation peaking between 0500 hr and 0700 hr. The lowest level of sleepiness coincided with the maximum tendency to wake up, and it was suggested that sleep termination may be closely related to the sleepiness/alertness rhythm.

Napping in shift work.

Two hundred eighty-two three-shift workers on rotating schedules filled out a questionnaire on napping behavior. Fifty-one percent were habitual nappers, but these workers rarely napped when working the afternoon shift or on days off. Four patterns were seen: non-napping (49%), morning shift napping (18%), night shift napping (18%), and both night- and morning shift napping (15%). The napping behavior was closely related to the length of the major sleep episode, which depended on the shift worked and on diurnal type; for example, morning shift nappers rated lower on a morningness/eveningness scale and night shift nappers higher. A study repeated 1.5 years later revealed that non-napping was a very stable behavior, whereas napping in many cases had disappeared, particularly among those who had been transferred to day work. The results indicate that for most shift workers napping compensates for sleep loss caused by the temporal displacement of sleep and modified by diurnal type.

Manifest sleepiness and the spectral content of the EEG during shift work.

Twenty-five three-shift workers in a process industry were subjected to ambulatory polysomnography during one afternoon and one night shift. The electroencephalographic (EEG) recordings were analyzed with spectral analysis. Subjective sleepiness increased during the night work but did not reach an extreme level. Five subjects fell asleep during night work and the involuntary naps were preceded by a few minutes of increased alpha (8-11.9 Hz) power density. Alpha and theta activity occurred in very short bursts. The hourly mean EEG alpha power density increased significantly but moderately during the night shift and correlated with subjective ratings of sleepiness. Theta power density (4-7.9 Hz) did not increase during the night shift, nor did it correlate with subjective sleepiness. It was suggested that the shift workers could prevent much of the polysomnographic manifestations of sleepiness by various types of activity (including succumbing to sleep). It was also suggested that averaging power density values across long time periods might not be an optimal strategy for detection of sleepiness, but rather some method of emphasizing the occurrence of alpha or theta bursts.

Morning work: effects of early rising on sleep and alertness.

The aim of the present study is to investigate how early morning work affects sleep and alertness. Twenty-two females, employed as airline cabin crew members, participated in the study. The design included two sleep conditions (work day and free day) for an early group and for a control group. The results show that early morning work reduced sleep to 5 hours and 12 minutes and that the reduction of sleep consisted of less stage 2 and rapid eye movement (REM) sleep. However, when the analysis was restricted to the first 5 hours, no differences in sleep stages, arousals, or sleep continuity were obtained between groups or conditions. Analysis of electroencephalogram (EEG) power density for the 0.5-16.5 Hz bands across nonREM periods showed no differences. With respect to the subjective ratings, early morning work was associated with more apprehension of difficulties in awakening and insufficient sleep. Daytime alertness and ease of awakening did not differ between groups, but the early group had significantly more sleepiness and complained more of unrefreshing sleep in connection with the work day compared to the free day. Ratings of insufficient sleep and high daytime sleepiness were mainly predicted (multiple regression analyses) by short total sleep time (TST), whereas apprehension of an unpleasant awakening was predicted by an early wake-up time. It was concluded that early morning work causes a reduction of sleep time and an increase in apprehension stress.

Relations between performance and subjective ratings of sleepiness during a night awake.

The present study validated the nine-point Karolinska Sleepiness Scale (KSS) and the new Accumulated Time with Sleepiness (ATS) scale against performance of laboratory tasks. The ATS scale was designed as a method for integrating subjective sleepiness over longer time periods. The subjects were asked if certain symptoms of sleepiness had occurred and, if so, for how long. Six subjects participated twice. Each time they were kept awake during the night (except for a short nap occurring during one of the nights in a counterbalanced order) and were tested at 2200, 0200, 0400 and 0600 hours. The tests included a 10-minute rest period, a 28-minute visual vigilance task and an 11-minute single reaction time task. KSS and visual analogue scale (VAS) ratings were given before each test, and ATS ratings were given after. Performance deteriorated clearly, and all three rating scales reflected increased sleepiness with time of night. Scores on the KSS and VAS showed high correlations with performance tasks (mean intraindividual correlations were between 0.49 and 0.71). Performance correlated even higher with the ATS ratings (r = 0.73-0.79). Intercorrelations between rating scales were also high (r = 0.65-0.86). It was concluded that there were strong relations between ratings of sleepiness and performance, that the ATS rating scale was at least as good as the other scales and that the ratings were affected by type of task.

Regulation of sleep and naps on an irregular schedule.

For many occupational groups there is an implicit requirement that sleep be taken according to very irregular schedules. The purpose of this study was to investigate the effects of an irregular sleep schedule on sleep and to determine if such effects could be attributed to the timing of sleep. Eight subjects slept in an isolation unit according to an irregular schedule comprising four 8-hour sleep periods, 12 6-hour sleep periods and 12 1-hour naps. The schedule was designed to give normal amounts of time (1/3 of total) in bed. Large variations in mean sleep efficiency (100-46%) were observed, which in individual cases frequently involved a complete absence of sleep. In 6-hour sleep periods the loss was due to an inability to maintain sleep, whereas in naps the loss was due to an inability to initiate sleep. The major causes of reduced sleep were short prior time awake and bedtime close to the circadian acrophase of rectal temperature (i.e. late afternoon). These two components accounted for 46% of the intraindividual variance of total sleep time (TST). TST increased by 9.2 minutes for each hour of prior time awake and by 10 minutes for each hour of deviation from the circadian acrophase of rectal temperature. Prior time awake also exerted a strong influence on slow-wave sleep (SWS), and circadian phase exerted a similar influence on rapid eye movement sleep. We concluded that irregular sleep patterns strongly affect sleep and that circadian and homeostatic influences are clearly discernible despite the irregularity.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of a short daytime nap after restricted night sleep.

Eight subjects participated on three occasions in a study investigating the effect of a 30 minute daytime nap opportunity on alertness/sleepiness. The baseline condition was a normal home sleep (7.5 hours, with bedtime at 2300 hours). Sleep during the other two conditions was between 2400 hours and 0400 hours. During one of the two 4-hour conditions, a short nap was allowed (between 1045 hours and 1115 hours). Self-ratings of sleepiness/alertness (Karolinska Sleepiness Scale) were recorded every hour. At 10, 12, and 15 hours, the subjects performed a 28-minute visual vigilance task. Electroencephalograms (EEG) and electrooculograms (EOG) were recorded continuously, including during a 10-minute standardized recording procedure at the beginning of each day. Mean total sleep time during the nap was 19.8 (standard error 2.4) minutes. Compared to baseline, EEG/EOG sleepiness and subjective sleepiness were significantly higher and vigilance performance at 10 hours lower, respectively, after the two short sleeps. The nap brought performance to baseline levels, and subjective sleepiness decreased significantly. It was concluded that the short nap had a clear positive effect on alertness.

Night driving, season, and the risk of highway accidents.

Official accident and traffic density statistics on Swedish highways were used to compute the relative risk (Odds Ratio - OR) of being injured or killed in a traffic accident at different times of day. After removing accidents due to alcohol 10344 accidents remained for computations, and the period 10:00h-11:00h was used as the reference point. The highest total risk was seen at 0400h (OR=5.7, Confidence interval = 5.6-5.8), with an OR of 11.4 (Ci=10.3-12.5) for fatal accidents at the same point. The same pattern was exhibited by single vehicle, head-on, and "other" (e.g., turning off the road) accidents, whereas overtaking and rear-end accidents did not show clear 24 hour patterns. Retaining alcohol-related accidents approximately doubled the nighttime peak for total accidents. During the winter, the peak of total accidents occurred at 03:00h (OR=3.8, Ci=3.5-4.0), five hours before sunrise, whereas the summer peak occurred at 04:00h (OR=11.6, Ci=11.3-11.9), shortly after the early summer sunrise and with consistently higher nighttime risk than for winter driving. It was concluded that early morning driving is several times more dangerous than driving during the forenoon. Apart from alcohol the effect seems related to sleepiness, but not to darkness.