SEASONAL COVARIATION OF THE CIRCADIAN PHASES OF RECTAL TEMPERATURE AND SLOW-WAVE SLEEP ONSET

There is a scarcity of well-controlled studies of the seasonal variati on in circadian rhythmicity. In the present study, the circadian phase of rectal temperature and the onset of slow wave sleep were studied i n a series of twelve 24-h experiments, one each month of the year, for six healthy subjects under controlled conditions in a climatic chambe r. In winter, as compared with summer, the average circadian rhythm of rectal temperature was phase delayed by 45 min, and the average onset of slow wave sleep was phase delayed by 40 min. The temporal relation ship between the circadian phase of rectal temperature and the timing of slow wave sleep was maintained throughout the year. Habitual rising and retiring times covaried as well. Furthermore, the circadian rhyth m of rectal temperature followed the timing of the photoperiod across the year, but had a much smaller range of seasonal variation. Apparent ly, the seasonal variation in the photoperiodic zeitgeber is largely c ompensated for by the stabilizing influence of secondary zeitgebers. H owever, in healthy subjects some effect of photoperiodic variation can still be observed.