24-HOUR MELATONIN AND CORE BODY-TEMPERATURE RHYTHMS - THEIR ADAPTATION IN NIGHT WORKERS

To determine whether the melatonin (MT) rhythm is adapted to a permane nt nocturnal schedule, 11 night workers were studied during their usua l 24-h cycle, and 8 day-active subjects during two 24-h cycles, once w ith night sleep and once after an acute shift of their sleep period to daytime. Rectal temperature (T-re) was continuously recorded. In day- active subjects, the MT rhythm was not affected by the acute shift in the sleep period, whereas the T-re rhythm was split in a biphasic patt ern with the circadian descending phase during the night of sleep depr ivation and a second descending trend during day sleep. Night workers showed a great variability in their MT profiles, with the onset of the MT release varying between 2145 and 0505. In contrast, the T-re rhyth m was homogeneously entrained to their usual sleep-wake cycle, with th e onset of the descending trend initiated before sleep onset so that t he large decrease was found, in some subjects, to be uncoupled with th eir MT increase. The night-active schedule did not induce any amplitud e modification of the T-re and the rhythms compared with day-active su bjects sleeping at night. No relationship between work-dependent facto rs and the extent of the MT shift could be found. These results show t he great variability in the timing of MT secretion among night workers , in contrast to the homogeneity of their T-re rhythm. The exact mecha nisms by which night workers adapt their circadian systems have not ye t been identified.