Twenty-four-hour melatonin and core body temperature rhythms: their adaptation in night workers

To determine whether the melatonin (MT) rhythm is adapted to a permanent nocturnal schedule, 11 night workers were studied during their usual 24-h cycle, and 8 day-active subjects during two 24-h cycles, once with night sleep and once after an acute shift of their sleep period to daytime. Rectal temperature (Tre) was continuously recorded. In day-active subjects, the MT rhythm was not affected by the acute shift in the sleep period, whereas the Tre rhythm was split in a biphasic pattern with the circadian descending phase during the night of sleep deprivation 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 Tre rhythm was homogeneously entrained to their usual sleep-wake cycle, with the onset of the descending trend initiated before sleep onset so that the large decrease was found, in some subjects, to be uncoupled with their MT increase. The night-active schedule did not induce any amplitude modification of the Tre and the rhythms compared with day-active subjects sleeping at night. No relationship between work-dependent factors and the extent of the MT shift could be found. These results show the great variability in the timing of MT secretion among night workers, in contrast to the homogeneity of their Tre rhythm. The exact mechanisms by which night workers adapt their circadian systems have not yet been identified.