Endocrine Abstracts

We carried out a series of experiments to identify common principles underlying the temporal organization of diverse circadian rhythms in endocrine and other functions in humans. Profiles of rhythms in plasma hormone levels, body temperature, and arousal state were measured while subjects remained at rest in continuous dim ( < 1 lux) light, after they had been exposed to artificial 'days' and 'nights' of different lengths. Subjects gave written informed consent for their participation in the studies, which were approved by the Institutional Review Board of the Intramural Research Program of the NIMH. Circadian rhythms were found to be organized in such a way that they exhibited distinct diurnal and nocturnal periods ('biological day and night') with abrupt transitions between the periods ('biological dawn and dusk'). Biological dusk was associated with abrupt transitions from periods of no melatonin secretion, rising core temperature and declining cortisol secretion, sleep propensity, REM sleep propensity, and EEG theta activity to periods in which the direction of these changes was reversed. Biological dawn was associated with abrupt changes in the opposite direction. Furthermore, the biological night (as indexed, e.g., by the duration of nocturnal melatonin secretion) became longer after individuals had been exposed to long artificial 'nights' than after they had been exposed to short artificial 'nights'. Thus, the human circadian pacemaker generates endogenous programs for a biological day and a biological night. These programs exhibit plasticity inasmuch as their duration can be adjusted to match seasonal changes in the solar day. Animal models suggest that this daylength-encoding mechanism could mediate effects of season on reproduction and mood in humans.