COMPARISON BETWEEN AUTORHYTHMOMETRIC METHODS AND A BASELINE MEASUREMENT OF CIRCADIAN RHYTHMS IN NIGHT-WORKERS

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1 J. Human Ergol.,11, Suppl.: (1982) COMPARISON BETWEEN AUTORHYTHMOMETRIC METHODS AND A BASELINE MEASUREMENT OF CIRCADIAN RHYTHMS IN NIGHT-WORKERS R. MOOD and G. HILDEBRANDT Institut fur Arbeitsphysiologie and Rehabilitationsforschung, Ketzerbach 21 1/2, 3550 Marsburg, ERG. 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. In general, biological rhythms represent spontaneous vital functions. They even persist if all influences of the outer environment are eliminated. Like other vital processes (i.e., metabolic rate, information processes), rhythmic parameters are deviated by stimuli from the inner and outer environment. Therefore, spontaneous biological rhythms can be observed only under strict constant conditions in the state of complete adaptation (HILDEBRANDT, 1981). In order to avoid so-called masking effects (ASCHOFF, 1960), the investigators already in the early times of biological rhythm research were anxious to study rhythmic functions under resting conditions including constant climate and equal distribution of food intake as well (MENZEL,1962). Although it was known that 385

2 386 R. MOOG and G. HILDEBRANDT in principle, reactions of the organism also exhibit a periodic rhythmic structure, WACHOLDER and BECKMANN (1952, 1953) and MENZEL (1962) showed the circadian rhythms to be superimposed by shorter periodic deviations being evoked by reactive loads on the organism, particularly by pathological stimulation. HILDEBRANDT (1962, 1981) pointed out the reactive periodicity to be characterized by 1) stimulus indication, 2) damping down of the amplitudes with increasing adaptation, and 3) preference of the whole number-related period durations to those at the spontaneous rhythms. The extension of field studies, particularly in the investigations of night and shift work, led to neglect of the old regulations, thus allowing masking effects to superimpose spontaneous 24 hr rhythms. It is, however, very difficult, or even impossible, to discriminate masking effects from spontaneous rhythmical functions because the responsiveness of the organism to disturbing effects itself undergoes rhythmical variations. MILLS et al. (1978) therefore tried to standardize masking effects by keeping the subjects awake 24 or more hours, in constant light, continuously in an upright position, active, and receiving small identical snacks hourly. By this, masking effects of sleep may be avoided. However, the continued upright position seems to be more unphysiological than a continued recumbent position. Furthermore, in shift work studies it would cause an additional sleep deficit for subjects who already suffer from sleep deficits due to shift work. We therefore decided to return to the previous mode of rhythm research and to follow up the circadian functions intermittently under complete resting conditions. This gives us the advantage of observing spontaneous sleep behavior, too, as an indicator of the natural sleep-wake-cycle. The handicap in field or experimental studies is, however, that it is impossible to control the undisturbed spontaneous rhythm and the activity rest cycle simultaneously. The aims of this paper are to estimate the extent of masking effects in shiftwork studies and to observe adaptation to night shift in experiments with reduced masking effects. Eight subjects with different circadian phase positions were controlled twice a week in a special noise-protected climatic chamber, where they remained on bedrest for 24 hr, receiving a low-protein diet and sleeping at their convenience. Sleep was controlled by means of EEG recordings, etc. Investigations were made within a period of 3 weeks of continuous night work. During working days, the subjects had to perform paper and pencil tasks between 2300 and 0800 hours. On some working days before or after control days, rectal temperature and pulse rate were continuously recorded. Figures 1 and 2 give some examples for different subjects. The first 8 hr of the control days are shown twice and used for comparison if necessary. In the night shift period and another period of 21 days of day shift, nyctohemeral pulserate levels were 14.7 beats/min higher compared to the control days. Especially at the beginning of the shift period, the curves exhibited no similarity compared to working days. Even when the subjects were sleeping or resting on working

3 AUTORHYTHMOMETRY AND BASELINE MEASUREMENT 387 Fig. 1. Comparisons between pulse rates collected under nyctohemeral and constant rest conditions during a night shift period of 21 days. days and ordered to rest and possibly sleep between 9 a.m, and at least 4 p.m., they did seldom reach pulse rates as low as they did on control days on comparable hours. Preceding activity may alter pulse rate, even if a person is sleeping. The differences between control and work days during resting time can reach the same level as the total amplitude of the circadian rhythm on control days. Moreover, the shapes of the curves taken under nyctohemeral conditions are so different from those on control days, that, for example, ultradian rhythms which play a role in adaptation to altered working schedules (HILDEBRANDT and STRATMANN, 1979) may not be detectable if the subject is allowed to be active. In short, if subjects are allowed to be active there are drastic masking effects in spite of a rather moderate activity. Hence, it is impossible to get a real picture of the spontaneous rhythms. Pulse rate is known to be strongly affected by activity, and rectal temperature

4 388 R.MOOG and G. HILDEBRANDT Fig. 2. Comparisons between rectal temperature collected under nyctohemeral and constant rest conditions within a night shift period of 21 days. is known to be less affected. Figure 2 compares rectal temperature on control and working days. Again the differences are striking. n the daily average, the difference between control days and nyctohemeral working days was 0.27 Ž, so rectal temperature is strongly affected by activity too, and spontaneous rectal temperature rhythms cannot be observed under nyctohemeral working conditions. A subject is able to override his sleep wakefulness rhythm. This motivation can derive from necessity as in the case of shift workers or in our experiments and will cause masking effects on the sleeping behavior. It is known that subjects with different individual circadian positions differ in their sleeping behavior (BENOIT and FoRET, 1981; BREITHAUPT et al., 1978) and in their tolerance to shift work (OSTBERG, 1973; HILDEBRANDT, 1980; MooG, 1981). Sleep wakefulness and temperature cycles are normally phase-locked (ZULLEIY and ASCHOFF, 1981). Thus differences in sleeping behavior including masking effects and the adaptive behavior of rectal temperature may be expected between morning and evening types adapting to an altered rest activity regimen.

5 AUTORHYTHMOMETRY AND BASELINE MEASUREMENT 389 Fig. 3. Mean masking effects in sleeping behavior of morning and evening types within a night shift period of 14 days (differences between the times falling asleep (A) and waking up (W) on control days and the following night shift days). In a respective study 8 other male subjects were chosen. The design was as before. In addition, however, the subjects received within the active period a physical training of 20 min each day. The results reported here were taken during a night shift period of 14 days. The differences between the onset and end of the main sleep episode under nyctohemeral and control circumstances are a measure of masking. Furthermore, they show the differences between spontaneous sleeping behavior and that forced by experimental conditions. In Fig. 3, evening types showed quicker decreasing masking effects during the night shift period compared to the morning types. This result indicates smaller differences between spontaneous sleeping behavior and forced sleeping behavior evening types, and is due to a quicker adaptation to night shift in evening types. This is demonstrated in Fig. 4, which shows sleep onset and end develop in the same way as the daily minimum does during the night shift period. Evidently, in respect to rectal temperature and sleeping behavior evening types do adapt while morning types do not. One morning type abruptly inverted his rectal temperature rhythm between the 10th and 14th day. His behavior may be explained by his special living conditions. In concordance, he showed clear ultradian rhythms in the first control day before the first night shift. In another group of 12 subjects, almost the same results could be found. These subjects were observed with the identical method in a period of 21 days of

6 390 R. MOOG and G. HILDEBRANDT Fig. 4. Mean courses in adaptation to night shift in morning and evening types in respect to sleeping behavior and rectal temperature minimum during a night shift period of 14 days. the night shift and a period of 21 days of the day shift but received no additional physical training. In both groups, evening types (n=10) shifted the mean of their daily minimum about 0.8 hr per day within the first 14 night shift days and reentrained with a speed of about 2.3 hr per day in the first three days of the following day shift period. These results are not in line with the hypothesis that control days may slow down the adaptation process essentially. Differences in adaptation behavior in both groups will be reported elsewhere. To sum up, differences between control days on nyctohemeral working conditions are drastic. It may be impossible to observe spontaneous rhythms if sub jects are allowed to be active, especially if subjects are adapting to an altered time regimen. Our method making use of constant resting conditions is able to discover the differences in adaptation behavior between morning types, known as shiftwork-intolerant, and evening types, known as shiftwork-tolerant.

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