The aim of this study was to compare the circadian variation of circulatory functions in physically average and very fit male dayworkers. Six physical very fit (group I, _O2 max 57.4ml/kg/min) and six average fit (group II, mean _O2 max 34.6ml/kg/min) railroad workers, mean age 32.1, were measured at 0200, 0600, 1000, 1400, 1800 and 2200 hours during a period of eight weeks. Mean group values were calculated according to the periodic regression analysis for harmonic intensities, and day and night mean values were compared with a paired t-test. In group I, heart rate at rest and at work and the ratings of perceived exertion were lower and the recovery of heart rate after work was faster both during the day and night than in group II. In the daytime, the reaction time was shorter in group I than in group II. At nighttime, the oral temperature was lower in group I than in group II. The amplitude of most of the variables was greater and the day-night differences became more pronounced in group I than in group II. The results indicate that there are rather constant differences in the physiological responses between the physically average and very fit male subjects throughout the circadian period. The lower responses to work, the faster recovery after work and the larger amplitude of physiological functions among very fit people suggest that they can better tolerate shiftwork.

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In this field study, we analyzed the effects of the permanent shiftwork system on sleep of 141 journalists working in a press agency. Their tasks were characterized by highly mental and cognitive activities. We found that dephasing between the sleep-waking cycle and internal biological rhythms was followed by an alternation of the quality and quantity of sleep. These effects increased with aging. A significant correlation was found between work amount during the evening shift (6.30 p.m.-0.30 a.m.) and the sleep onset time. A descriptive model is proposed in which the performance rhythm interacts with the circadian rhythm of biological variables which might control sleep.

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Self recorded data for 28 days of day and night sleep were analysed in 33 experienced, permanent night workers employed as night guards in a security company. They worked either ten 16 hr shifts (group SG10, n=22) or fourteen 12 hr shifts (group SG14, n=6), during 4 weeks, or eighteen 9 hr shifts of beat patrols (group BP18, n=5). The mean day sleep after each shift of group SG10 lasted 3.6 hr, but the sum of day and night sleeps on the same day amounted to 12.9 hr. Group SG14 subjects worked blocks of 2-3 consecutive shift, day sleep between shifts being 6.5 hr and longer than that after the last shifts of 5.2 hr. For subjects in group BP18 who worked blocks of 2-4 consecutive shifts, day sleep between shifts was 7.1 hr, longer than day sleep after the last shifts of 6.4 hr. The cumulated day and night sleep after the last shifts of the latter two groups amounted to 14.5 hr. When working blocks of 3-4 consecutive night shifts, the data indicated the possibility of accumulation of sleep deficit on days between shifts. The length of day sleep after the last shift increased significantly in the order of group SG 1 O; SG 14, and BP 18, parallel to the increase in the number of consecutive night shifts. This supports the assumption that temporary sleep deficits accumulate in shift systems using blocks of more than 2-3 consecutive night shifts.

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It is difficult to have day sleep of sufficient length after night work due to circadian rhythms and various conditions disturbing sleep. Thus, some night workers divide their day sleep into two or more parts, while the others have a single day sleep. It is of interest to know which sleep patterns would favor the recovery from fatigue. In this study, night workers were classified according to the frequency of the day sleep, i.e., those who usually had day sleep twice a day (split sleep group) and those who usually had a continuous day sleep (continuous sleep group). The two groups were compared concerning the number of fatigue symptoms complained of before and after work fatigue. Permanent night workers in the central market of Osaka and shift workers of an iron and steel works participated in this investigation. For the permanent night workers, the total duration of day sleep of the two groups was equally short (about 5.3 hr), but the number of fatigue symptoms of the continuous sleep group was more than the split sleep group, especially after work. In the case of the shift workers, however, the duration of day sleep of the split sleep group was longer than that of the continuous sleep group. It was suggested that day sleep split into two may be more effective for recovery than a continuous day sleep.

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This paper examines the sleep strategies practiced by shiftworkers and explores sleep variable interventions which might reduce the incidence of sleep complaints among shiftworkers. For analysis purposes, a day during the workweek can be segmented into four periods of time : work, sleep, an optional and shorter sleep period or naps, and off time. An analysis of survey data from shiftworkers indicates that the morning, afternoon/evening, and night shifts result in differences for these periods in sequence, length, and time-of-day. The sequence for day shift workers is sleep, work, nap, off time, for afternoon/evening workers sleep, off time, nap, work, and for night workers sleep, off time, nap, work. Night shift workers have the shortest sleep period, the longest nap period, sleep during the day, have a higher incidence of napping, and sleep after rather than before work. Thus, as a group they show a high incidence of sleep complaints. However, night shift workers who do not report napping show a significantly lower incidence of sleep complaints than those who report nap times. The data suggest a number of possible sleep variable interventions : limiting night shift work to natural short sleepers, recommending minimal napping while on the night shift, changing the usual sequence of periods to allow sleep at a more favorable time-of-day, and maintaining the same sequence for all shifts.

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The structure of many shiftwork systems currently in operation reflects tradition rather than design. Research into shiftwork, however, suggests that parameters related to objective physiological, psychological, and social criteria can be included in the design of shift systems, but that they nonetheless meet mathematical and other constraints. To illustrate the existing diversity of shift systems, the frequency distribution of some fundamental characteristics of 487 continuous shift plans in operation within the Federal Republic of Germany are shown. These characteristics include the number of consecutive night shifts, start and finish times of shifts, duration of shifts, distribution of leisure time, duration of the shift cycle, and the regularity of the shift system. They are discussed with respect to research evidence concerning the following criteria: physiological adaptation; performance and accidents; wellbeing (e.g., sleep, fatigue, and appetite); health (e.g., gastrointestinal and psychosomatic disorders); personal and social problems. Nine practical recommendations are suggested as design parameters for planning shift systems.

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Usually, the control of circadian functions in subjects who continue active behavior is rather complicated by so-called masking effects arising from reactive changes of the respective functions to the individual activity cycle, meal cycle, etc. In order to minimize those disturbing masking effects, 8 subjects were controlled in a special climatized and noise-protected chamber where they remained on bedrest for 24 hr, receiving a low-protein diet and sleeping at their convenience. The results were compared with autorhythmometric measurements taken from identical subjects performing certain activity schedules one day before or after the chamber control. All investigations were done within a period of 3 weeks of continuous night work and another 3 weeks of day work. The results show drastic differences in rectal temperature and heart rate between both methods of circadian rhythm control. It is concluded that masking effects play an important role in the hitherto contradictory evaluation of circadian adaptation to night and shift work. Differences in speed and quality of adaption were found between so-called morning and evening types by means of baseline measurement.

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Common problems of both women and men doing shift work are caused by disturbed circadian rhythms and sleep deficit, while individual adaptation to shift work is different in various workers with the same design of work hours. Ageing often gives rise spontaneously to earlier wakingup time, and more difficulties in falling asleep. In a study of Swedish railroad workers, mean sleep time was shorter in men already over 40 years of age. Studies concerning women workers in heavy industrial shift work are scanty. We studied 600 women in steel plants. Attitudes to work, social disturbances, illnesses, sleep pattern, and fertility outcome were studied. As a whole no-adverse health effects could be observed, though the study was cross-sectional. A couple of studies have examined nurses doing night work; the total hours of work are reduced in this work. In married women with small children, an extra load of household work added to the industrial work gave rise to special problems. A good practical solution, both for elderly and women workers, has been found in our Swedish study, 50% working hours arranged as full-time work every second week. This kind of arrangement would be easy to follow in work organization and shift rotation schemes.

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For 5 weeks, 15 shift workers (5 working on continuous and 10 on semi-continuous shifts), 15 day workers, and 6 former shift workers, all between 50 and 56 years old, registered daily the times they went to sleep and awoke and the sleep interruptions. The mean duration of sleep per 24 hr over the 5 weeks was 7 hr 52 min for the day workers and 7 hr 43 min for the former shift workers. For the continuous shift workers, the mean duration during the night shift weeks was 6 hr 31 min, during the afternoon shift week 7 hr 55 min, and during the morning shift week 6 hr 55 min. For the semi-continuous shift workers, the values were 6 hr 30 min for the night shift week, 7 hr 10 min for the afternoon shift week, and 6 hr 50 min for the morning shift week. Between the different groups there were no clear differences in sleep interruptions. On the average, these self-selected shift workers got nearly one hour less sleep per night than the day workers. Detailed study of their sleep charts suggests that they are reasonably well adapted to their shift system.

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