Ageing, physical fitness and shiftwork tolerance

ooo34870(95)ooo46-1

Applied

Ergonomics

Vol 27, No.

I, pp. 2529,

1996

Else&r Science Ltd Printed in Great Britain COO3-6870/96 $10.00 + 0.00

ELSEVIER

Ageing, physical fitness and shiftwork tolerance Mikko HQrmti Institute of Occupational Health, Department 0~’ Physiology, Helsinki, Finland

The effects of ageing and physical fitness on shiftwork tolerance are reviewed. Ageing is one of the most cited factors decreasing the health of shiftworkers. Although long-term prospective studies on ageing are few, shiftworkers over 40-45 years of age seem to sleep worse after night, hut not after Imorning shifts. Sleepiness after successive night shifts is also decreased by age although older shiftworkers’ ability to resist acute sleep loss seems to be even better. The reasons for the altered sleep and wakefulness of older shiftworkers are probably related to changes in circadian rhythms, especially higher ‘morningness’. Sleep need may also decrease with age which could explain some of the differences found in sleep length. Physical fitness as a factor increasing tolerance to shiftwork is a recent finding. Although the effects of physical activity on slee:p have been studied in detail, the relationship of physical fitness to sleep is still a controversial issue. In shiftworkers, moderate physical training has been shown to increase sleep length and night-time alertness. It has not been shown, however, that exercise would quicken the circadian adjustment to night work. It is recommended that work time arrangements should take account of the older workers’ changing personal references. Continuous night work should be voluntary after 40 years of age. Moderate physical exercise a few hours before the main sleep period is recommended.

will review recent advances on the relationships of ageing, physical fitness and tolerance to shift work.

Most shiftworkers have at least occasional disturbances of sleep, while about one-third complain of fatigue, gastrointestinal and nervous problems (Akerstedt, 1988; Waterhouse et al, 1992). The existence of shiftworkers’ health cornplaints is highly individual (Htirmti, 1993). While our knowledge on the principle health effects of shiftwork is now on a good basis, there is still a growing need ILOfind evidence on relevant coping mechanisms for different groups of shiftworkers. The concept of shiftwork tolerance was introduced by Andlauer and Reinberg (Andlauer et al, 1979) in their studies of shiftwork and health. Shiftwork tolerance was defined as the existence of the most common subjective health effects of shiftwork: the existence of sleep-wake disturbances, digestive and neurological problems (i.e. persisting sleepiness and unusual irritability). This concept of shiftwork tolerance is discussed in this paper (social consequences of shiftwork are not covered), focused on two individual factors affecting shiftwork tolerance and especially the sleep of shiftworkers: ageing and physical fitness. General research interest on ageing has grown since the numbers over 45 years will approximarely double in many OECDcounties in the future (Ilmarinen, 1991). Physical fitness is a rather new, but relevant issue in relation to possible coping mechanisms in shiftwork. The paper

Ageing There are great inter-individual differences in the subjective health of shiftworkers (HBrmti, 1993). One of the factors most cited as decreasing shiftwork tolerance is ageing. However, long-term prospective studies on ageing and shiftwork are extremely rare and found only on selective topics, like incidence of ischaemic heart disease (Knutsson et al, 1986). Since the available cross-sectional studies are also often based on subjective instead of objective measures of health, we could actually question the validity of current knowledge on the effects of ageing on shiftworkers’ health. Responses to questionnaires are based on the subjects’ own perception of what is normal. This perception may very well differ between shift- and day workers, as well as between groups of different ages. A 65year-old night worker may feel that he is actually rather well (in relation to his own criteria) if he can sleep 4 hours after a night shift. Secondly, there are strong indications that the need for sleep may change with ageing and if that is true, differences in sleep length and structure are not the whole story. Relevant questions would be: which are the age-specific changes

25

Ageing, physical fitness and shiftwork tolerance: M. Hiirmii

26 in shiftworkers’ ability?

alertness,

Shift- and age-dependent shiftwork

performance

differences

and

work

in tolerance to

There are some well-cited cross-sectional questionnaire studies showing that after the age of 40-50 years the health risks and/or sleep disturbances in shiftwork start to increase (Foret et al, 1981; Koller, 1983; Akerstedt and Torsvall, 1981). Koller (1983) studied the health of 300 matched cases of permanent shift workers, day workers and drop-outs corresponding in age and work experience. She found a sharp decline in several aspects of health in shiftworkers after the age of 40-45 years. The decline was indicated by increases in absence due and cardiovascular gastro-intestinal to sickness, diseases, as well as sleep disturbances. Sleep, which is the major problem area of most shiftworkers, was studied in more detail by Foret et al, 1981, and Akerstedt and Torsvall, 1981. Both studies indicated that shiftworkers over 40 years of age reported more often shorter sleep length and poorer subjective sleep quality. Also, both age and longer shiftwork experience were independently related to poorer sleep. Akerstedt and Torsvall (1982) pointed out, however, that it was actually only the day sleep (after night shifts) which was curtailed by age. In fact, he reported that ‘older individuals slept longer than younger ones before the morning shift’. In a field study electroencephalographic (EEG) recordings, with Torsvall et al (1981) characterised in more detail the observed differences in the day sleep of young and older shiftworkers. Older shiftworkers’ day sleep included relatively more stage shifts, awakenings, stage 1 sleep, a higher diuresis and a higher noradrenaline excretion during the sleep. Recently, we have studied the sleep characteristics of ‘three-shift’ workers in relation to age and different starting times of the shifts (Mulder et al, 1994). The results showed that, unrelated to the slightly different timing of the shifts (06 or 07, 14 or 15, 22 or 23), the older workers fell asleep easier, slept longer, rated the quality of sleep higher and were significantly less tired compared to their younger colleagues during the morning shifts. After night shifts, the older shiftworkers slept shorter, as shown before, but there were no differences between the groups in sleepiness during the night shifts. If the ending time of the night shift was later (07 compared to 06), the older shiftworkers had a tendency to wake up subjectively ‘too early’ during the day sleep compared to younger shiftworkers. Mechanisms The reasons for the age-specific changes in the sleep and wakefulness of shiftworkers may be several. Firstly, the normal change in sleep structure seems to occur at a chronological age far earlier than most agerelated declines in other biological functions (Bliwise, together with the age-specific 1993). This change, changes in circadian rhythms and especially the preferred timing of sleep could explain some of the observed changes. ‘Morningness’ means an advanced circadian rhythm of sleep-wakefulness and activity. In general, ageing

correlates to earlier times of retiring, rising and having meals, as well as a lack of flexibility in sleep patterns and an advanced motor activity rhythm (Renfrew et al, 1987; Lieberman and Wurtman, 1989; Minors et al, 1989; Monk et al, 1991). Elderly people (over 65 years of age) seem to possess a circadian rhythm of physical activity which is already 2 hours phase-advanced compared to young people (Lieberman and Wurtman, 1989). Shiftworkers, although their circadian activities and sleep are strongly influenced by the timing of shifts, also show remarkable age differences in their ‘morningness’ (Akerstedt and Torsval 1981; Mulder et al, 1994). The higher morningness can explain why it is relatively easy for an older shiftworker to fall asleep in time and thus have enough sleep before an early morning shift. On the other hand, since the end of the day sleep is also regulated by circadian factors (Folkard, 1994), the greater morningness of an aged shiftworker can explain why he or she sleeps less after a night shift. In a field study of hospital nurses, morningness was found to be the most important individual factor decreasing sleep length after night shifts, while in morning shifts morningness was related to better sleep (Harma et al, 1988a). It has been suggested that morningness is related to shorter endogenous circadian periodicity (e.g. Minors and Waterhouse, 1990). Also, ageing has been found to decrease the amplitude of many circadian rhythms and to increase the tendency to internal desynchronisation (Van Good, 1986). These chronobiological changes due to ageing probably explain why older shiftworkers seem to adjust more slowly to consecutive night shifts (Matsumoto and Morita, 1987; Harma et al, 1994). Although we could not find significant age differences in the circadian adjustment of hospital nurses based on oral temperature measurements in the field (Harma et al, 1990), there was an age difference in the speed of adjustment in a controlled laboratory study (Harma et al, 1994). Although older shiftworkers clearly show more sleep disturbances during day sleep, there is no clear evidence that ageing would increase symptoms of sleepiness during night shifts. If that were true, it could be that the sleep need of older shiftworkers is decreased. In fact, there are now several reports showing that younger subjects are remarkably more sensitive to an acute sleep loss than middle-aged or older subjects (Froberg et al, 1972; Bonnet and Rosa, 1987; Brendel et al, 1990; Monk et al, 1992). For example, according to the data of Monk et al (1992), it was apparent that the 24-hour means performance, mood and activation of elderly people did not decrease as much during night hours as did those of younger subjects. We have studied recently the effects of age on both sleepwakefulness and endogenous circadian rhythms of shiftworkers during consecutive night shifts (Harm% et al, 1994). Experienced letter sorters were studied in a sleep laboratory under closely controlled conditions. Younger shiftworkers were found to be more sleepy than the older shiftworkers during the first night shift. However, during the third night shift, the older shiftworkers were already more sleepy than the young shiftworkers. The significant interaction effect of age and the number of consecutive night shifts on sleepiness

Ageing, physical fitness and shiftwork tolerance: M. Hiirmii

were explained by the faster circadian adjustment of the younger shiftworkers, as studied by the circadian rhythms of body temperature and salivary melatonin. Finally, ageing changes. greatly the social and family situation of a shiftworker. Older shiftworkers tend to have better housing (and sleeping) conditions, fewer domestic responsibilities, and more experience of the coping mechanisms about how to manage best with working in abnormal shifts. These differences, which seem to favour older shiftworkers in relation to health, cannot explain the shorter sleep length of older shiftworkers after night shifts, but they may explain why young or inexperienced shiftworkers often have serious problems with working in shifts. Practical considerations

What can be done to help older shiftworkers? As recommended by the new IL0 convention and recommendations (International Labour Office, 1990), working time arrangements should, as far as possible, take into account workers’ personal preferences for the scheduling of their free-tirne. These preferences change with age. Older shiftworkers usually prefer earlier starting times of the shifts than younger shiftworkers. The need for and distribution of free days may also change. Work periods of several consecutive days may accumulate the sleep disturbances of older shiftworkers, since their basic capability to adjust during night shifts is impaired. We recommend in the first place that continuous night work should be voluntary after 40 years of age. In practice, continuous night work could be voluntary for everybody. To be able to find the shiftworkers with greater medical problems, regular health checks should be arranged for all shiftworkers after the age of 40. In these health checks emphasis should be laid on the history of the change of sleep disturbances and/or gastrointestinal symptoms in relation to working in shifts. Finally, transfer from night to day work, when necessary, should be arranged for health reasons (Table I). Physical fitness The possibility of finding individual life styles or relevant coping mechanisms promoting adaptation to shiftwork is important, since we cannot avoid shiftwork in the modern society. On the other hand, although shift scheduling is the most important single action, it is not enough to minimise the medical problems of shiftwork. Physical fitness as a factor increasing tolerance to shiftwork is a rather recent finding. On the other hand, the effects of physical activity on sleep have been studied in detail.

Table 1 Recommendations:

ageing and shiftwork

(1) Working time arrangements should, as far as possible, take into account workers’ changing personal preferences, Older shiftworkers normally prefer earlier starting times than younger shiftworkers and dislike night shifts. (2) Continuous night work should be voluntary after the age of 40. (3) Regular health checks should be arranged after 40 years of age. Emphasis should be laid on the history of sleep disturbances and/ or gastrointestinal symptoms while working in shifts.

27 Physical fitness, exercise and sleep

In general, physical activity seems to be connected with sleep, tiredness and other different types of symptoms (Shapiro and Bachmayer, 1988; Urponen et al, 1988; Stevenson and Topp, 1990). The questionnaire of Urponen et al (1988) was submitted to an age-stratified random sample of 1600 Finnish subjects aged 36-50 years. Exercise, such as jogging and evening walks, was the most frequently reported factor for promoting sleep and improving its quality. Thirty three per cent of the men and 30% of women reported exercise as the most important factor for improving sleep. The most often perceived positive effects of exercise on sleep were ‘easier to fall asleep’ (70%), ‘deeper sleep’ (66%) and ‘better feeling in the morning’ (65%). The positive effects of exercise on sleep were more frequent when the exercise was performed in the early evening (before 8 p.m.) than late at night (after 8 p.m.). In a large number of laboratory studies using EEG methods, the relationship between exercise, sleep and the level of physical fitness of the subjects has been shown to be more complicated than in the studies using subjective ratings of sleep. Two of the three past reviews of the area generally support the hypothesis that physical exercise will increase both sleep duration and the amount of slow wave sleep [SWS] (Horne, 1981; Torsvall, 1981). SWS is believed to be essential for the brain restoration and recovery, and therefore is of special interest. In the latest review of the area, which also covers some later studies, Trinder et al (1988) conclude that, while some evidence exists that exercise does have indirect effects on sleep structure, there is not evidence that physical activity would directly cause longer or structurally more restorative sleep. Only a very few laboratory studies have compared subjects before and after a fitness training programme to test the hypothesis that physical training would increase SWS sleep. Shapiro et al (1984) studied the sleep of young untrained individuals at the beginning, middle and end of an 18 week army basic training programme. SWS was shown to increase on the second and third measurement occasions. In another study (Paxton et al, 1983) the athletes had higher levels of SWS than the sedentary individuals irrespective of the changing physical fitness level of the athletes. Vitiello et al (1994a; b) recently studied the effects of aerobic and stretching/flexibility training on the sleep and circadian rhythms of older adults (mean age 66 years). Aerobic training caused an increase of stage 3-4 sleep by 33% and the participants reported a subjective improvement of overall sleep quality and sleep latency. Aerobic training resulted also in shifts in the amplitude, phase and sinusoidal shape of the circadian temperature rhythms of the healthy older adults towards those observed in healthy younger individuals. Horne (1981) has suggested that the critical factor in determining the effect of exercise on SWS is whether or not the exercise induces an increase in brain temperature. The increased brain temperature would enhance brain metabolism and result in a compensatory increase in SWS. The body heating theory could also explain some of the discrepancies found in the studies

28 of physical exercise and sleep. The timing of the body heating by the exercise has thus been found to be critical as to whether the heating produces an increase or decrease in the amount of SWS (Horne and Shackel, 1987). Whether or not the passive body heating theory can explain some of the discrepancies of the literature on exercise and sleep, there must also be some additional mechanism by which exercise can influence sleep. The additional mechanisms may be of psychological or endocrinological nature. It is also surprising how little attention has been paid to the possible effects of exercise on alertness/sleepiness after sleep. Physical fitness and shiftwork tolerance Since the circadian variation is one of the main mechanisms explaining tolerance to shiftwork, it could be questioned whether there are any basic differences between fit and unfit people in their circadian rhythms. To answer the question we studied the circadian variation of different physiological functions in physically very fit and average fit subjects (Hsrmti et al, 1982; Htirm5 and Linsimies, 1985). The physically very fit subjects had lower heart rates at work, less perceived exertion, better orthostatic tolerance and faster recovery after physical work not only during the day, but also at night. We also found significantly higher oral temperatures and sleepiness circadian amplitudes in the physically very fit subjects, and interpreted it as a possible sign of better shiftwork tolerance. To test the hypothesis of better shiftwork tolerance due to exercise, a follow-up study with a matched-pair design was designed to examine the effects of moderate physical training on fatigue, sleep, performance and the psychosomatic symptoms of the shiftworkers (Htirrng et al, 1988b; 1988~). One hundred and fifty one women were selected from a group of about 400 nurses in a large hospital. The subjects were aged 20-49 years and had at least 1.5 years experience in shift work. The physical fitness programme consisted of four months of moderate training two-six times per week including mainly jogging, walking, swimming and gymnastics. Physical fitness, subjective symptoms, sleep and physiological adjustment to night work were studied before and after the intervention. Due to the intervention_, the training group achieved significant increases in VOzmax (+6%) and muscle strength (+22%), and a significant decrease in HR at rest (five beats), while no significant changes took place in the control group. According to the questionnaire, the training group reported a remarkable decrease (from 21% to 4%) in general fatigue and sleepiness ‘during the last three weeks’, while the general fatigue of the control group did not change significantly (from 27% to 20%). However, when the changes in alertness were measured separately in different shifts, alertness decreased only in the night shifts, but increased in the evening shifts. Sleep length increased 12-24 minutes after all shifts. The intervention also caused a significant decrease in musculoskeletal symptoms, especially symptoms of the back. The field measurement of body temperature, alertness and performance at every 2 hours confirmed that the training programme decreased sleepiness during the night shift. There were, however, no significant changes in body temperature circadian adjustment

Ageing, physical fitness and shiftwork tolerance: M. Hiirmii Table 2 Recommendations:

exercise and shiftwork

(1) Moderate physical exercise should be preferred instead of intensive training. (2) Exercise should be performed several hours before the main sleep period. (3) In relation to shifts, the best times for exercise are after a morning or a day shift, and when performed after a night shift, before an evening nap.

from the morning to the second night shift when the change of the body temperature circadian rhythm (phase and amplitude) was examined. Short-term memory performance improved significantly in the training group during both the morning and night shifts, but the change was not significant when compared to a (smaller) increase also in the control group. Our findings suggest that moderate physical training has mostly beneficial effects on sleep, fatigue and the performance of shiftworkers. Since the exercise was performed at different times of the day and on different days of the week, the effects of regular exercise at exact times of the day on the circadian adjustment still remain to be studied. There are indications that regular exercise at exact times of the day can synchronise the circadian rhythms in laboratory animals. Also, physical work during day-time synchronises the circadian rhythm of body temperature to a greater extent compared to mentally active or no-work conditions (Kobayashi et al, 1991). Practical considerations We recommend regular physical exercise for shiftworkers (Table 2). There are, however, several practical problems to implement regular training programs in shiftworkers. Regular training in teams may be difficult due to irregular working times and it can be difficult to organise group exercise or to book practice times for teams. Also, disturbances of sleep-wakefulness cycles and reduced free-time may themselves reduce any motivation for exercise. In particular, young women with families may have difficulties finding personal free-time for exercise. In any case, regular exercise should be planned in relation to the individual sleep-wake schedules of the workers. On the other hand, the optimal times of the day for exercise should also be determined by the nature and intensity of exercise, environmental conditions and the individual phase types [morningness] (Reilly, 1990). For shiftworkers, we recommend that moderate physical exercise should be preferred to intensive training. The exercise should be performed at least 3-4 hours before the main sleep period. Intensive exercise immediately before a shift should be avoided if the work tasks require very high alertness. The best times for exercise are after a morning or a day shift and, if performed after a night shift, some hours before a late evening nap.

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