Appropriate use of bright light promotes a durable adaptation to night-shifts and accelerates readjustment during recovery after a period of night-shifts

Abstract

The present study examines the stability of bright light circadian readjustment during two consecutive dim light night-work periods and circadian synchronization during the recovery after a night-shift period. A sample of 10 subjects was divided into 2 groups (control group: 6 subjects; experimental group: 4 subjects). All subjects worked during 5 days, between 23:00 and 07:00 h and then went to sleep. Subjects received 2500-3000 lux between 02:00 and 05:00 h during 5 days for the control group and 3 days for the experimental group. During recovery after the night-shift periods, three cycles of bright light were administered at two different times: 12:00-15:00 h for two of six subjects from the control group and 10:00-13:00 h for all subjects (4) of the experimental group. By the fifth cycle of night-work the maximum of urinary aMT6s excretion that occurs at 05:00 h in the baseline condition was shifted to 12:00 h for the control and experimental groups (delay in hours: 7±1.6 (control); 7±1 (experimental)). This result suggests that three cycles of bright light are sufficient to induce a significant phase delay and that this delay remained stable when night-work proceeded under dim light. The phase delay of the circadian aMT6s excretion by exposure to bright light was accompanied by an improvement of the quality of day sleep and level of cognitive and psychomotor performances for control and experimental groups. No significant difference was observed in the two groups for daytime sleep and nocturnal performance. The two bright light periods used during the three days of recovery induced a complete synchronization in five of six subjects. One subject showed a partial synchronization probably because he remained at the laboratory under dim light during the day and had few family and social contacts.     

A 14-h night-shift in the control room of a fire brigade

All 29 operators in the control room of a fire brigade performed test sessions during work and leisure time at 2-h intervals on a day with a 14-h night-shift, a day off and a day with a 10-h morning shift. The test sessions consisted of a visual search choice reaction time test and two Sternberg memory search tests (1 or 5 letters had to be memorized). Furthermore, the operators recorded their oral temperature and their subjective alertness before each test session. The subject also kept a diary of work and sleep times for 14 days. The circadian rhythm of body temperature was hardly changed by a single night-shift. Parallel to the body temperature, the subjective alertness fell significantly during the night-shift reaching a minimum value at 06:00 h. The mean reaction times increased non-significantly at the end of the night-shift and the results of the two Sternberg memory search tests showed no significant trends. The sleep of the operators after the night-shift was on average reduced to 5 hs 34 min. The results of subjective rating of alertness and reaction time test are interpreted as effects of the combined influences of circadian rhythmicity, sleep loss and time on professional task. Most results support the conclusion that a 14-h night-shift without extensive breaks is not acceptable.     

A 14-h night-shift in the control room of a fire brigade

All 29 operators in the control room of a fire brigade performed test sessions during work and leisure time at 2-h intervals on a day with a 14-h night-shift, a day off and a day with a 10-h morning shift. The test sessions consisted of a visual search choice reaction time test and two Sternberg memory search tests (1 or 5 letters had to be memorized). Furthermore, the operators recorded their oral temperature and their subjective alertness before each test session. The subject also kept a diary of work and sleep times for 14 days. The circadian rhythm of body temperature was hardly changed by a single night-shift. Parallel to the body temperature, the subjective alertness fell significantly during the night-shift reaching a minimum value at 06:00 h. The mean reaction times increased non-significantly at the end of the night-shift and the results of the two Sternberg memory search tests showed no significant trends. The sleep of the operators after the night-shift was on average reduced to 5 hs 34 min. The results of subjective rating of alertness and reaction time test are interpreted as effects of the combined influences of circadian rhythmicity, sleep loss and time on professional task. Most results support the conclusion that a 14-h night-shift without extensive breaks is not acceptable.     

A 14-h night-shift in the control room of a fire brigade

Abstract

All 29 operators in the control room of a fire brigade performed test sessions during work and leisure time at 2-h intervals on a day with a 14-h night-shift, a day off and a day with a 10-h morning shift. The test sessions consisted of a visual search choice reaction time test and two Sternberg memory search tests (1 or 5 letters had to be memorized). Furthermore, the operators recorded their oral temperature and their subjective alertness before each test session. The subject also kept a diary of work and sleep times for 14 days. The circadian rhythm of body temperature was hardly changed by a single night-shift. Parallel to the body temperature, the subjective alertness fell significantly during the night-shift reaching a minimum value at 06:00 h. The mean reaction times increased non-significantly at the end of the night-shift and the results of the two Sternberg memory search tests showed no significant trends. The sleep of the operators after the night-shift was on average reduced to 5 hs 34 min. The results of subjective rating of alertness and reaction time test are interpreted as effects of the combined influences of circadian rhythmicity, sleep loss and time on professional task. Most results support the conclusion that a 14-h night-shift without extensive breaks is not acceptable.     

The after-effects of a prolonged period of day-sleep on subjective sleep quality

The effects of day-sleep on the quality ofsleep are not restricted to the night-work period itself. Effects can be measured during night-sleep in the period following night-work. A study on these after-effects, using a self-report instrument to measure sleep quality, is described. A total of N=7O operators wcrc observed over five consecutive undisturbed nights after a working period of seven morning shifts (N= 35) and after a working period of seven night-shifts (N= 35). The quality of sleep during the third night after the night-shift period was still significantly worse than the third night after the rnorning-shift period. No differential (after-)effects were observed with respect to differences in sleep duration.     

Effects of sleep inertia on cognitive performance following a 1-hour nap

Abstract

This study investigated the effects of a 1-h nap on subsequent performance in spatial memory (SM) and logical reasoning (LR) tasks. The objective was to evaluate the duration of the effects of sleep inertia (SI). The performance was measured in two independent groups of subjects. The 1-h nap took place at 00:00 h in group 1 and at 03:00 h in group 2. In each task, the experimental design comprised one no-nap condition in which subjects had no sleep before the night tests, and a nap condition that comprised the 1-h nap followed by the test sessions. To measure the duration of SI effects, the subjects were tested in two 30-min sessions and the data in each session were analysed in sub-units of time of 3 min each. In each task the results showed no effects on accuracy, and no circadian effects of napping were found. In each task, analyses of pooled data of the two groups showed that the performance in the 1-h nap condition exhibited significant reductions of speed immediately following awakening, when compared with no-nap, reflecting SI effects. In SM, the slowing lasted 24 min, and 27 min in LR. Practical implications of the results are discussed.     

Effects of sleep inertia on cognitive performance following a 1-hour nap

This study investigated the effects of a 1-h nap on subsequent performance in spatial memory (SM) and logical reasoning (LR) tasks. The objective was to evaluate the duration of the effects of sleep inertia (SI). The performance was measured in two independent groups of subjects. The 1-h nap took place at 00:00 h in group 1 and at 03:00 h in group 2. In each task, the experimental design comprised one no-nap condition in which subjects had no sleep before the night tests, and a nap condition that comprised the 1-h nap followed by the test sessions. To measure the duration of SI effects, the subjects were tested in two 30-min sessions and the data in each session were analysed in sub-units of time of 3 min each. In each task the results showed no effects on accuracy, and no circadian effects of napping were found. In each task, analyses of pooled data of the two groups showed that the performance in the 1-h nap condition exhibited significant reductions of speed immediately following awakening, when compared with no-nap, reflecting SI effects. In SM, the slowing lasted 24 min, and 27 min in LR. Practical implications of the results are discussed.     

Psychophysical conditions and hormonal secretion in nurses on a rapidly rotating shift schedule and exposed to bright light during night work

The psychophysical conditions of 15 young female nurses, working in a rapidly-rotating shift system (2-2-2-2), modified according to some psychophysiological criteria, and exposed to short period (4 × 20 min) of bright light (2350 Lux) during their night duty, were studied in order to evaluate their adaptation to night work and to test a possible positive effect on it of bright light. Subjective evaluations of work load and psychophysical conditions, performance measures, hormonal excretion (cortisol, 6-sulphatoxymelatonin and catecholamines), oral temperature and activity-sleep logs were taken during the shiftwork cycle, comprising one morning, one afternoon and two consecutive nights under normal and bright light. The results showed that this rapidly-rotating shift system had little effect upon the normal circadian rhythms of the body. Moreover, the lengthening of the night shift to 10 h can be considered to be acceptable, provided that work load is reduced and there are sufficient rest pauses available. On the other hand, the reduction to 7 h of the length of the day shifts and delayed start of the morning shift to 07:00 h appeared to be convenient both in relation to the work load and sleep duration. Some positive effects of bright light upon psychophysical conditions and performance efficiency were noted, while hormonal excretion and body temperature did not show any effect of bright light; in particular, melatonin excretion was not suppressed appreciably by the bright light used.     

Psychophysical conditions and hormonal secretion in nurses on a rapidly rotating shift schedule and exposed to bright light during night work

Abstract

The psychophysical conditions of 15 young female nurses, working in a rapidly-rotating shift system (2-2-2-2), modified according to some psychophysiological criteria, and exposed to short period (4 × 20 min) of bright light (2350 Lux) during their night duty, were studied in order to evaluate their adaptation to night work and to test a possible positive effect on it of bright light. Subjective evaluations of work load and psychophysical conditions, performance measures, hormonal excretion (cortisol, 6-sulphatoxymelatonin and catecholamines), oral temperature and activity-sleep logs were taken during the shiftwork cycle, comprising one morning, one afternoon and two consecutive nights under normal and bright light. The results showed that this rapidly-rotating shift system had little effect upon the normal circadian rhythms of the body. Moreover, the lengthening of the night shift to 10 h can be considered to be acceptable, provided that work load is reduced and there are sufficient rest pauses available. On the other hand, the reduction to 7 h of the length of the day shifts and delayed start of the morning shift to 07:00 h appeared to be convenient both in relation to the work load and sleep duration. Some positive effects of bright light upon psychophysical conditions and performance efficiency were noted, while hormonal excretion and body temperature did not show any effect of bright light; in particular, melatonin excretion was not suppressed appreciably by the bright light used.     

Auditory evoked potentials as a function of sleep deprivation

Event-related brain potentials (ERPs) were studied in subjects deprived of sleep over a 48-h test period to assess the effects of different durations of continuous wakefulness on ERP components and to determine whether changes in the ERP components were related to changes in performance. Forty subjects were randomly assigned to either an experimental (sleep deprived) group (n = 30) or a control (not sleep deprived) group (n = 10). For the experimental subjects, ERP and performance measures were obtained in four-h test blocks throughout the 48-h period. Performance was assessed using the Walter Reed performance assessment battery. The control subjects were tested at the same times except during designated sleep periods. Both performance and evoked potential measures showed systematic changes over the experimental test period in association with sleep deprivation, time of day, and repeated testing. The latency of the N2 component of the evoked potential covaried with throughput measures on the performance assessment battery across the 12 four-h test blocks of the experiment. These data suggest that ERPs reflect central processes that change across the sleep deprivation period and that ERP measures might be useful in assessment and prediction of performance degradation under adverse conditions such as sleep loss.     

Combined work stresses: Effect of reduced air renewal on psychological performance during 72h sustained operations

Human tolerance limits for sustained operations under the combined stress of stale air and sleep loss were studied in two groups often male volunteers over a 72 h period, with only 1h of sleep permitted after 32, 48 and 60h. The experiment investigated whether such sustained performance might be additionally influenced by mild hypoxia (15 vol% O₂) together with correspondingly increased carbon dioxide levels (5 vol%).

Performance on various psychological tests showed the expected decrease with increasing duration of sleep loss. However, there were no clear cut differences in performance between the control and the hypoxia groups. There were, however, more pronounced decreases with time in either group in the more complex tasks as compared to simple reaction time and vigilance tasks. Short-term memory improved probably due to learning. The missing effect of hypoxia could be attributed to a hyperventilation response in the experimental subjects and an increase in cerebral blood flow initiated by the hypercapnia.     

Shiftwork tolerance and circadian rhythms in oral temperature and heart rate

Abstract

The study aimed at establishing differences in the circadian rhythm parameters of oral temperature and heart rate between three groups of workers matched by age: tolerant shiftworkers with good sleep quality and few psychosomatic and digestive complaints, intolerant shiftworkers with poor sleep quality and more psychosomatic and digestive complaints, and workers who had never worked in shifts. The study was performed at an oil refinery recreation centre where the subjects were isolated for three days. During that period they lived under constant conditions, i.e. their activities, timing and content of meals, ambient temperature and light were controlled. Hourly measurements of oral temperature and heart rate were performed over a 24-h period. The parameters of the physiological rhythms were estimated by means of the COSINA method. Generally, the differences between the groups were low and not statistically significant. The authors could find no difference in the amplitude and phase position between the tolerant shiftworkers unlike other authors. The difference between these results and those of others is likely to be due to the fact that earlier studies were performed in field conditions where many factors that could have masked the results were not controlled for.     

The benefits of a nap during prolonged work and wakefulness

Abstract

Prolonged work scenarios with demands for sustained performance are increasingly common. Because sleep loss inevitably compromises functioning in such situations, napping has been proposed as a countermeasure. The optimal timing of the nap relative to its benefits for performance and mood is not known, however. To address this issue, 41 healthy adults were permitted a two-hour nap at one of five times during a 56-hour period of intermittent work, with no other sleep. Naps were placed 12 hours apart, near the circadian peak (P) or trough (T), and were preceded by 6(P), 18(T), 30(P), 42(T), or 54(P) hours of wakefulness. Work test bouts occurred every few hours and consisted of a variety of psychomotor and cognitive tasks as well as mood scales completed at the beginning, middle and end of each bout. A total of eight performance and 24 mood parameters were derived from the bouts and compared between groups at all test points prior to and following the naps. An estimate of the extent to which each nap condition differed from the control (P54) condition was derived by totalling the proportion of test points that yielded statistically significant results relative to the total number of tests conducted both before and after naps.

Although all performance and most mood parameters displayed a circadian-modulated deterioration as the protocol progressed, a nap appeared to attentuate the extent of this change in all performance parameters but not in mood parameters. Overall, the timing of the nap across days and within the circadian cycle was irrelevant to its effect on performance, suggesting that it diminished the intrusion of sleepiness into behavioural functioning, even though subjects were phenomenally unaware of this benefit.     

The benefits of a nap during prolonged work and wakefulness

Prolonged work scenarios with demands for sustained performance are increasingly common. Because sleep loss inevitably compromises functioning in such situations, napping has been proposed as a countermeasure. The optimal timing of the nap relative to its benefits for performance and mood is not known, however. To address this issue, 41 healthy adults were permitted a two-hour nap at one of five times during a 56-hour period of intermittent work, with no other sleep. Naps were placed 12 hours apart, near the circadian peak (P) or trough (T), and were preceded by 6(P), 18(T), 30(P), 42(T), or 54(P) hours of wakefulness. Work test bouts occurred every few hours and consisted of a variety of psychomotor and cognitive tasks as well as mood scales completed at the beginning, middle and end of each bout. A total of eight performance and 24 mood parameters were derived from the bouts and compared between groups at all test points prior to and following the naps. An estimate of the extent to which each nap condition differed from the control (P54) condition was derived by totalling the proportion of test points that yielded statistically significant results relative to the total number of tests conducted both before and after naps.

Although all performance and most mood parameters displayed a circadian-modulated deterioration as the protocol progressed, a nap appeared to attentuate the extent of this change in all performance parameters but not in mood parameters. Overall, the timing of the nap across days and within the circadian cycle was irrelevant to its effect on performance, suggesting that it diminished the intrusion of sleepiness into behavioural functioning, even though subjects were phenomenally unaware of this benefit.     

Perceived exertion under conditions of sustained work and sleep loss

The rated perceived exertion (RPE) for treadmill walking carrying a 22 kg pack was measured in two studies with heart rate, workload and psychological measures. Study 1 used an initial treadmill speed and grade producing 40% of maximal oxygen consumption. Exercise subjects walked 1/2-h, then did other tasks for 20-h for 16 sessions on two 20 h long days. Group I had a 4 h nap, while Group 2 had 4 h of rest between workdays. Subjects maintained the 40% workload throughout the two days with a 3 h nap between days, one group starting exercise at midnight and one at noon.

A steady increase in RPE over the sessions each day was found in each study. There were no differences in RPE between the Nap and Rest groups in Study 1 or between the midnight and noon groups in Study 2. RPE was higher on Day 2 in each study. RPE correlated significantly with heart rate, distance walked or elevation at most time periods on the first workday in Study 1, but not Study 2. RPE was positively correlated with fatigue, symptoms, sleepiness and negative mood, and negatively correlated with vigor at most of the time periods day 1 in Study 2; with similar significant correlations at only one time period on day 1, Study 1. RPE was lower the first session Day 2 as compared to the last session Day 1 in both studies. This pre-post break recovery did not differ between groups in either study.     

Perceived exertion under conditions of sustained work and sleep loss

Abstract

The rated perceived exertion (RPE) for treadmill walking carrying a 22 kg pack was measured in two studies with heart rate, workload and psychological measures. Study 1 used an initial treadmill speed and grade producing 40% of maximal oxygen consumption. Exercise subjects walked 1/2-h, then did other tasks for 20-h for 16 sessions on two 20 h long days. Group I had a 4 h nap, while Group 2 had 4 h of rest between workdays. Subjects maintained the 40% workload throughout the two days with a 3 h nap between days, one group starting exercise at midnight and one at noon.

A steady increase in RPE over the sessions each day was found in each study. There were no differences in RPE between the Nap and Rest groups in Study 1 or between the midnight and noon groups in Study 2. RPE was higher on Day 2 in each study. RPE correlated significantly with heart rate, distance walked or elevation at most time periods on the first workday in Study 1, but not Study 2. RPE was positively correlated with fatigue, symptoms, sleepiness and negative mood, and negatively correlated with vigor at most of the time periods day 1 in Study 2; with similar significant correlations at only one time period on day 1, Study 1. RPE was lower the first session Day 2 as compared to the last session Day 1 in both studies. This pre-post break recovery did not differ between groups in either study.     

Polyphasic sleep strategies improve prolonged sustained performance: A field study on 99 sailors

In situations where continuous prolonged work demands exist, the habitual nocturnal monophasic (6-8h duration) sleep pattern can rarely be accomplished, and performance effectiveness may sometimes be severely compromised by accumulation of sleep debt. Several studies have shown that naps can be disproportionately effective in recovering functioning during continuous work (CW).

In this study, sleep-wake patterns and their relationship to performance were studied for 99 sailors involved in solo and double-handed ocean sailing races (a model of a highly demanding CW situation). Most sailors spontaneously adopted multiple nap sleep-wake schedules and adapted without major difficulties to such polyphasic patterns. 66.5% had mean sleep episode durations (SEDs) ranging from 20 min to 2h. Overall mean Total Sleep Time (TST) per 24h was reduced from a baseline of 7.5 to 6.3h. Race performance correlated negatively and significantly with mean SEDs and TSTs. Best performance results were obtained by those sleeping for periods of between 20 min and 1 h and for a total of 4.5 to 5.5h of sleep per day.

The results are discussed together with several chronobiological, phylogenetic and experimental studies and issues, all of which suggest that adult humans may have a damped polyphasic sleep-wake tendency. It is also proposed that polyphasic sleep schedules could become promising and feasible solutions for the management of sleep requirements under prolonged CW situations.     

Dealing with shift work: Physical fitness, temperature, and napping

This paper reviews the effects of generally controllable factors such as physical conditioning, ambient temperature, and amount of prior sleep in adjustment to night work periods and shift work. One can expect a 5-10% decline in capacity for work in nocturnal work periods as compared to daytime work periods. This decreased capacity would dissipate if workers maintained a consistent sleep-wake routine for 8-16 days after moving to a new shift. Proven means for accelerating this adaptation are currently not available. The ability to perform work also declines as the length of the work period increases but depending upon individual tasks. Physical conditioning improves mood and general well-being, but no strong evidence currently indicates that conditioning increases tolerance or adjustment to shift work.

Increased ambient temperature increases the stress of work, although studies do not address heat as a factor in adjustment to shift work. While lower nocturnal temperature would be assumed to reduce heat stress during night shifts, supporting data does not exist. Studies have not addressed the negative consequences of cold stress or of rotating from night to day shifts with added heat stress.

The proper use of short sleep periods either as preparation for or as a response to a shift change can ameliorate some effects of shift rotation. Data indicate that performance on 'graveyard' shifts can be maintained close to baseline levels following true prophylactic naps while performance may decline by up to 30% when such naps are not taken. While there is evidence that naps or even rest periods without sleep are beneficial in improving mood in normal young adults, these data do not apply to 'replacement' naps. Studies of interjected naps imply that such naps do reduce sleep debt but do not imply that such naps are more beneficial than longer sleep periods. Naps appear to be most advantageous when the accumulated sleep debt is least.     

Health, well-being and burnout of ICU nurses on 12- and 8-h shifts

It is generally agreed that some features of shift systems can influence the extent of well-being and health problems experienced by the workers involved. Extended working days (9-12 h) have been found to aggravate some problems associated with shiftwork, especially when the work is mentally and emotionally demanding. The aim of the study was to compare measures of health, sleep, psychological and social well-being, job satisfaction and burnout of ICU nurses on 12- and 8-h shifts. The groups of subjects were matced for age, length of shiftwork experience, marital status and number of hours worked. the 12-h shift nurses, when compared to their 8-h shift colleagues, experienced more chronic fatigue, cognitive anxiety, sleep disturbance and emotional exhaustion. Job satisfaction seems to be independent of the shift duration. The nurses on 12-h shifts reported less social and domestic disruption than those on 8-h shifts. The 12-h shift nurses showed worse indices of health, well-being and burnout tan the 8-h shift nurses. It is suggested that this may be associated with their longer daily exposure to the stress of work. The increased number of rest days of 12-h shift nurses seems tobe insufficient to dissipate the adverse health and well-being effects that built up over their longer shifts.     

Is there an optimum number of night shifts? Relationship between sleep, health and well-being

By concentrating on the impact of a specific shift-system feature on the well-being of those concerned, rather than on the impact of the shift system as a whole, one might be able to offer more meaningful suggestions as to what constitutes a better form of shift system. The present study focused on the impact of the number of consecutive night shifts worked on the health and well-being of two groups of nurses (permanent night and rotating shift). All nurses completed a copy of the Standard Shiftwork Index, which is a set of questionnaires designed for comparing the effects of different types of shift system on large groups of workers. It includes measurements of psychological ill-health, physical ill-health, chronic fatigue, social and domestic disruption, attitudes towards shiftwork, sleep quality and sleep habits. Results showed clearly the impact of the number of consecutive nights worked on health and well-being, not directly, but indirectly through the impact on sleep duration and sleep quality. Sleep duration was shown to increase with more consecutive nights worked. This in turn was found to predict sleep quality, which in turn was found to be the stronger direct predictor of psychological and physical ill-health: i.e. better health was associated with longer and better quality sleeps. Explanations in terms of circadian adaptation are discussed.