Re-entrainment of Body Temperature in Experimental Shift-Work Studies

In a series of experimental shift work studies six subjects worked on continuous night shift for three weeks, four subjects worked on a 1-1-1 shift system (1st day morning shift, 2nd day afternoon shift, 3rd day night shift, 4th day off), and two subjects worked on an 2-2-2 shift system (the ‘metropolitan rota’). Rectal temperature was continuously recorded in each experiment.

The greatest changes in the circadian rhythm of body temperature occurred on the second night shift day, and during the first week of continuous night shift working. The changes involved both shifts in the phasing of the rhythm, and alterations in its form. The re-entrainment of the rhythm to its normal phasing took two or more days after two or more successive night shifts, but only one day after a single night shift. Considerable individual differences in the rhythm adjustment to night work were observed.

In a field validation of these findings, 34 shiftworkers in municipal gas and water supply undertakings measured their oral temperature every 2h both on and off duty. Although only very few temperature readings were obtained during sleep periods, the results confirmed, in general, those of the experimental studies.     

What is the preferred number of consecutive night shifts? results from a crossover intervention study among police officers in Denmark

Among police officers in Denmark, we studied (i) how many consecutive night shifts participants preferred at baseline; (ii) preferences regarding three intervention conditions (two, four, and seven consecutive night shifts followed by the same number of days off/day shifts: ‘2 + 2’, ‘4 + 4’, ‘7 + 7’) at follow-up; (iii) characteristics of participants preferring each of these intervention conditions. Questionnaire data from a crossover intervention study were used (baseline: n = 73; follow-up: n = 68). At baseline, 49% preferred four consecutive night shifts. At follow-up, 57% preferred ‘4 + 4’, 26% preferred ‘2 + 2’ and 26% preferred ‘7 + 7’. Participants, who preferred longer spells of night work experienced that night work was less demanding, found it easier to sleep at different times of the day, and were more frequently evening types compared with participants who preferred shorter spells of night work. The participants’ preferences are likely to be influenced by their previous shift work experience.

Practitioner Summary: We investigated police officers’ preferences regarding the number of consecutive night shifts. The majority preferred four consecutive night shifts. Those who preferred the longer spells of night work found night work less demanding, found it easier to sleep at different times of the day, and were more frequently evening types.     

Field Studies of Shiftwork: I. Temporal Patterns in Psychophysiological Activation in Permanent Night Workers

Abstract

The diurnal variation in body temperature, catecholamine excretion, performance, and subjective alertness was studied among 24 permanent night workers during the 1st, 3rd, and 5th day of their work ‘ week ’of six nights. Most of the variables showed a significant diurnal pattern with high values during work. Furthermore, the patterns of variation were similar during the three nights of measurement without any observable adjustment over the week. It was therefore suggested that permanent night work might facilitate a more rapid switch over to night work after free days.     

Post-lunch nap as a worksite intervention to promote alertness on the job

A worksite study was conducted to examine whether a 15-min nap during a post-lunch rest period would affect subsequent alertness, performance, and nocturnal sleep in eight factory workers under a 3-week protocol. Subjects were asked to take the nap at 12:30?h on a reclining chair during the nap week, and to remain awake during the no-nap week. The order of these 2 weeks was counterbalanced between the subjects. During the third, follow-up week, each subject determined whether or not she/he would nap. Alertness on the job and nocturnal sleep were assessed using a sleep diary. Wrist activity was also recorded during sleep at night. Choice reaction time task (RT) was performed at 10:00 and 15:00?h every day of the nap week and every other day of the no-nap and follow-up weeks. Perceived alertness was significantly higher in the afternoon after nap than after no nap at the end of the week. Similar effects were observed during the follow-up week where almost half of the subjects napped. No significant differences between the three weeks were found for RT performance or nocturnal sleep. Workers' attitudes toward the nap were favourable. Although further intervention research is required, our results suggest that post-lunch napping may have the potential to promote daytime alertness at work.     

Post-lunch nap as a worksite intervention to promote alertness on the job.

A worksite study was conducted to examine whether a 15-min nap during a post-lunch rest period would affect subsequent alertness, performance, and nocturnal sleep in eight factory workers under a 3-week protocol. Subjects were asked to take the nap at 12:30 h on a reclining chair during the nap week, and to remain awake during the no-nap week. The order of these 2 weeks was counterbalanced between the subjects. During the third, follow-up week, each subject determined whether or not she/he would nap. Alertness on the job and nocturnal sleep were assessed using a sleep diary. Wrist activity was also recorded during sleep at night. Choice reaction time task (RT) was performed at 10:00 and 15:00 h every day of the nap week and every other day of the no-nap and follow-up weeks. Perceived alertness was significantly higher in the afternoon after nap than after no nap at the end of the week. Similar effects were observed during the follow-up week where almost half of the subjects napped. No significant differences between the three weeks were found for RT performance or nocturnal sleep. Workers' attitudes toward the nap were favourable. Although further intervention research is required, our results suggest that post-lunch napping may have the potential to promote daytime alertness at work.     

Night shift paralysis in air traffic control officers

Abstract

In an earlier paper (Folkard et al. 1984) we reported on the incidence of a temporary but incapacitating paralysis known as ‘night shift paralysis’. This appeared to be a special form of sleep paralysis that occurs when night workers manage to maintain a state of wakefulness despite considerable pressures to sleep. The incidence of this paralysis might thus be assumed to reflect the level of sleep deprivation associated with different shift systems or individuals. The present survey was designed to examine this possibility in a sample of 435 Air Traffic Control Officers (ATCOs) from 17 different countries who were on a variety of different work schedules. The incidence of this paralysis was found to be affected by four main factors, all of which might reasonably be assumed to influence the night-worker's level of sleep deprivation or sleepiness. These were the time of night, the number of consecutive night shifts, the requirement to work both a morning and a night shift starting on the same day and individual differences in the flexibility of sleeping habits. These results suggest that the incidence of this paralysis may indeed prove to be a useful ‘critical incident’ for comparing the level of sleep deprivation associated with different shift systems or individuals.     

Alertness of night nurses: two shift systems compared

The alertness of hospital trainee nurses during their night shift work was assessed by recording performance at night on an arousal-sensitive unprepared simple reaction time task. One group carried out a number of separate weeks on night shift throughout their three-year course. Here performance fell from first to seventh day of the week on night shift, implying progressive sleep deprivation. Another group covered their night work by a single three-month 'permanent' night shift of four (or three) nights on and three (or four) days off each week. Here initial performance level fell by the 45th night but had returned to normal by the last (90th) night. In both systems individuals varied considerably in their ability to maintain performance during sustained night work. These results strengthen the case for permanent night shifts, with careful selection of personnel, as a means of organizing night nursing in hospitals.     

The effects of frequently rotating shiftwork on sleep and the family life of hospital nurses

The effects of three frequently rotating shifts in an irregular sequence on the daily activities of 239 Japanese female hospital nurses were studied by the time-budget method. The nurses recorded their daily activities for several consecutive days. The questionnaire was returned by 80·8% of the participants, and recordings of 1016 days were analysed. A two-way analysis of variance clarified that the shift combination influenced the daily activities. The most distinct result was that nurses spent significantly more time on free-time activities on the day when they worked the night shift followed by the evening shift than they did on the day when they worked any other shift combination. Nurses offset sleep deprivation either by sleeping during the day before and after working the night shift (82–100%) or by sleeping 2 to 4?h later in the morning after working the evening shift and on days off. There was a strong positive correlation between total sleep time (including day sleep) and the length of the interval between two consecutive shifts (r = 0·95, p < 0·001). This result suggests that more than 16 h between work shifts is required to allow more than 7?h of total sleep time. In an analysis by household status, nurses who had young children (average age, 2·8 years) slept less and spent less time on free-time activities than did other nurses.     

Sleep in a live-in mining operation: The influence of start times and restricted non-work activities

The amount of sleep obtained between shifts is influenced by numerous factors including the length of work and rest periods, the timing of the rest period relative to the endogenous circadian cycle and personal choices about the use of non-work time. The current study utilised a real-world live-in mining environment to examine the amount of sleep obtained when access to normal domestic, family and social activities was restricted. Participants were 29 mining operators (26 male, average age 37.4 ± 6.8 years) who recorded sleep, work and fatigue information and wore an activity monitor for a cycle of seven day shifts and seven night shifts (both 12 h) followed by either seven or fourteen days off. During the two weeks of work participants lived on-site. Total sleep time was significantly less (p < 0.01) while on-site on both day (6.1 ± 1.0 h) and night shifts (5.7 ± 1.5 h) than days off (7.4 ± 1.4 h). Further, night shift sleep was significantly shorter than day-shift sleep (p < 0.01). Assessment of subjective fatigue ratings showed that the sleep associated with both days off and night shifts had a greater recovery value than sleep associated with day shifts (p < 0.01). While on-site, participants obtained only 6 h of sleep indicating that the absence of competing domestic, family and social activities did not convert to more sleep. Factors including shift start times and circadian influences appear to have been more important.     

Adaptation to permanent night work: the number of consecutive work nights and motivated choice

To study the effects of different patterns of days on and days off duty within a permanent night shift schedule, we studied two groups of female permanent night nurses, one (n=42) working‘many’ nights in a row, and the other (n=36) working ‘few’ nights in a row. The groups were matched for several important moderating variables. All nurses were interviewed individually, and they answered several questionnaires. A MANOVA revealed no statistically significant differences in subjective health between the two groups. Nor was any difference found for satisfaction measures. The many-night group however seemed to put more effort into dealing with their schedule; they used more active coping, had a higher cigarette consumption, and engaged more in sport. They also stayed one hour less in bed during the day after the last night shift. It is concluded that neither schedule was a clear threat to subjective health because the acceptance of permanent night work was the result of a motivated choice and because social support was high. The differences between the two schedules were not large enough to suggest that one of them should be favoured over the other.     

A shipboard study of a four-crew rotating watchkeeping system

Abstract

Body-temperatures and subjective alertness ratings were obtained at 2-hourly intervals in a shipboard study of six subjects watchkeeping on a rapidly rotating system with a 4-day cycle. The results were compared with those from an earlier study of this system conducted on land in ‘laboratory’ conditions. The similarity of the temperature curves from the two studies indicated that prolonged exposure to shipboard conditions made no difference to the underlying circadian rhythm in this variable, which appeared to be near-normal in form. Subjective alertness showed a clear rhythm of considerable amplitude; the phasing of this rhythm suggested that, under the present system, operational effectiveness is likely to be reduced in watches held in night hours. Further analyses indicated that it was also likely to be reduced in daytime hours following these night watches, because of the disruption of sleep that results from them. An alternative ‘fixed hours’ watchkeeping system is proposed to overcome these unwanted effects.     

Experimental Studies of Shift-Work II: Stabilized 8-hour Shift Systems

Abstract

Thirty-one subjects were employed in an experiment to determine whether the relationship between efficiency at mental tasks and the circadian rhythm of body temperature observed in an earlier study was affected by an increase in the length of the duty-spell from 4 to 8 hours. Subjects wore assigned either to a control ‘ day ’ shift (O800–1600), a ‘ night ’ shift (2200–0600) or a ( morning ) shift (0400–1200), and were tested for a period of 12 consecutive days on the same shift. The control shift-workers showed no consistent effects of fatigue due to the increased length of the duty-spell. Adaptation of temperature rhythm to work on the night shift was only partial, but was relatively closely reflected in the recorded performance trends. Very little adaptation to work on the morning shift was observed, and performance was thought to have been affected by partial sleep deprivation. It was concluded that body-temperature was as effective a predictor of overall mental efficiency in most industrial-type shifts as in the special 4-hour shift system previously investigated.     

Sleep at Home and in the Sleep Laboratory: Disturbance by Recording Procedures

Abstract

Nineteen healthy men, aged 20 to 55 yr, who did not suffer from insomnia had their sleep recorded by EEG and EOG in the laboratory for 3 to 12 nights each over periods of 1 to 4 weeks. They also gave subjective reports each day which proved to be valid although not always very accurate assessments of their sleep latency, the number of mid-night awakenings and the times of sleeping in the laboratory. There was some ' adaptation ' to the laboratory over the first 2 nights, but awakenings during the night continued for up to 12 nights to be reported more than twice as frequently in the laboratory than at home. They were apparently often caused by intermittent traction of electrodes on the face and scalp as the subjects turned over in bed From the point of view of mid-night awakenings, EEG/EOG recordings are unlikely to give an accurate assessment of the subject's usual sleep, even after several ‘ adaptation ’ nights.     

Short and Long-term Adjustment of Circadian Rhythms in ‘ Permanent ’ Night Nurses

Two studies are described in which differences were found in the adjustment of the circadian rhythms of full and part-time night nurses to night work. A distinction was drawn between short-term adjustment that takes place over successive night shifts, and long-term adjustment that may take the form either of a permanent ‘flattening’ of the rhythm, or of a facilitation of short-term adjustment. The results from the first study indicated that even when the potential for greater short-term adjustment was controlled for, the full-time staff showed greater adjustment to night work. The second study examined these differences in long-term adjustment in greater detail. No evidence was found of a permanent ‘flattening’ of the full-timers’ circadian rhythms, although they showed clear evidence of adjustment even on the first of a period of successive night shifts. The full-timers also showed more evidence of adjustment from the first to the second night shift. It is suggested that these differences in long-term adjustment may reflect differences in the degree to which the nurses scheduled their lives towards night work.     

Field Studies of Shift Work at an Austrian Oil Refinery I: Health and Psychosocial Wellbeing of Workers who Drop Out of Shiftwork

Most of the problems that arise in shift workers prove to be social or family-bound rather than strictly medical, though health may in fact be affected seriously by these problems. The present study was concerned with the question whether, and in what way, men giving up shift work (‘ drop-outs ’) differ from those who stay on shift work, and from those on permanent day work. A questionnaire containing items about personal, family, and social life, and health, working conditions, and sleep was given to 270 workers in an oil refinery. All respondents were subsequently interviewed.

Taking all the results together it is concluded that shift and night work leads, in a proportion of cases, to difficulties with regard to work and family life, as well as in social and health matters. The results from the ‘drop-outs’ group indicate that in certain individuals very long lasting ‘ psychosomatic’, ‘pseudoneurotic’ or ‘sensitisation’ reactions may develop.     

Experimental Studies of Shift-Work I: A Comparison of ‘ Rotating ’ and ‘ Stabilized ’ 4-hour Shift Systems

Abstract

Efficiency at ‘ mental ’ tasks was observed when performed according to a time schedule imposed by following one of two different 4-hour shift systems for a period of 12 consecutive days. Twenty-eight subjects were assigned either to a ‘ rotating ’ system, in which each 4-hour period of the 24 hours was worked once every 72 hours in a repeating cycle, or to a ‘ stabilized ’ system, in which the work periods were from 1230 to 1630 and 2400-0400 each day. In the rotating system, alterations in the level of several aspects of performance at different times of day were found to be related quite closely to concurrent fluctuations in body temperature arising from its natural circadian rhythm. A shift in the phase of this rhythm in response to the now sleep/waking cycle imposed by the stabilized system was accompanied by a corresponding change in the relative levels of performance observed in the two work periods. Thus in both systems body temperature was, in effect, a predictor of performance efficiency. Some implications for the organization of shift working are discussed     

Inversion of the Sleep Wakefulness Pattern: Effects on Circadian Variations in Psychophysiological Activation

Abstract

Thirty-six habitually dayworking railway repairmen were exposed to a 3 week period of nightwork. The subjects were studied with respect to circadian rhythms in catecholamine excretion, body temperature, subjective alertness and mood. For half the group the measurements covered one 24 h period before nightwork, the first week of night work, the third week of night work, and the first week after return to day work. For the other half measurements were made during the first and third day week after the night work. During day work weeks all variables exhibited pronounced circadian variation, peaking in the early afternoon, with the exception of body temperature which reached its maximum in the evening. During the first week of night work the day-oriented pattern of adrenaline excretion persisted but the mean 24 h level was increased and day sleep levels were very high. By the third week of night work thecircadian pattern had flattened out at a very low mean level. For noradrenaline excretion considerable adjustment (comparable to an inversion) to night work was seen with high night values. For body temperature, selfrated alertness and mood circadian functions flattened out during night work. It was concluded that all variables were strongly affected by the exposure to night work and that adrenaline excretion indicated a stress response of the organism. With reference to other studies it was also concluded that adrenaline excretion is not easily phase-shifted through a three week spell on night work, while noradrenaline in contrast appears to adjust very rapidly.     

Sleep in a live-in mining operation: The influence of start times and restricted non-work activities

The amount of sleep obtained between shifts is influenced by numerous factors including the length of work and rest periods, the timing of the rest period relative to the endogenous circadian cycle and personal choices about the use of non-work time. The current study utilised a real-world live-in mining environment to examine the amount of sleep obtained when access to normal domestic, family and social activities was restricted. Participants were 29 mining operators (26 male, average age 37.4 ± 6.8 years) who recorded sleep, work and fatigue information and wore an activity monitor for a cycle of seven day shifts and seven night shifts (both 12 h) followed by either seven or fourteen days off. During the two weeks of work participants lived on-site. Total sleep time was significantly less (p < 0.01) while on-site on both day (6.1 ± 1.0 h) and night shifts (5.7 ± 1.5 h) than days off (7.4 ± 1.4 h). Further, night shift sleep was significantly shorter than day-shift sleep (p < 0.01). Assessment of subjective fatigue ratings showed that the sleep associated with both days off and night shifts had a greater recovery value than sleep associated with day shifts (p < 0.01). While on-site, participants obtained only 6 h of sleep indicating that the absence of competing domestic, family and social activities did not convert to more sleep. Factors including shift start times and circadian influences appear to have been more important.     

Tolerance to Shift of Sleep,as Related to the Individual's Circadian Phase Position

The differential adaptation of ‘morning’ and ‘evening’ types to changes in sleep schedules was investigated in two studies. The first was carried out on six female subjects, classified as morning or evening types on the basis of their rhythms of body temperature, and on their answers to a morningness-eveningness questionnaire. Bed times on single nights were varied between 2100h and 0300 h. On each night measurements were taken of latency of sleep onset and of sleep duration. Body temperature, heart rate and respiratory rate were also recorded, as was self-estimated vigilance during the day following the experimental sleep. Evidence of sleep deficiency after late bed limes was found in the morning but not in the evening types; the latter, however, had longer sleep latencies after early bed times than the morning types.

In the second study eight male subjects underwent the same basic experimental treatment, but in this case sleep was monitored for three consecutive nights with a given bed time, in order to examine adaptive processes. The overall results were similar to those of the first study. However, it was also found that, whereas the duration of sleep tended to increase over the three consecutive nights with later bed times in all subjects, this was more marked in the evening than in the morning types.     

Gender,ageing, and shiftwork intolerance

The study investigated the hypothetical differences between male and female shiftworkers in their susceptibility to shift work-related health and social problems, with the special reference to the role of the age factor. The comparison concerned two matched-for-age-and-occupation groups of men and women, each of 83 persons, selected from the larger studied population of more than 700 workers in a Polish steel plant. The subjects were crane-operators employed in the same forward-rotated, three-shift, four-team shift system, 4:4:4 with shift changes at 06:00, 14:00, 22:00; and 48 h off following each shift block. The investigation comprised a battery of questionnaires on demographic characteristics, sleep quantity and quality, subjective health complaints, and opinions on shiftwork. The analysis of data revealed that men slept more than women, especially when working on the afternoon and night shifts. The differences became more striking and significant for all work shifts and days-off when related to declared individual sleep requirements. Women experienced more sleep disturbances than men and suffered more frequently from drowsiness during work, especially when working the morning shift. The ratings of subjective health were lower in women, with exception of respiratory complaints. Women generally suffered more than men from symptoms considered as specific to the ‘intolerance syndrome’, i.e. psychoneurotic, digestive, circulatory, and those of chronic fatigue. However, after passing the ‘critical decade’ of 40–50 years their subjective health generally improved, whereas in males one observed the consequent deterioration of health with advancing age. Women more often complained about their health and went to see the doctor, but on the other hand, they did not tend to quit shiftwork as often as did their male counterparts.