BIMODAL PATTERNS OF HUMAN MELATONIN SECRETION CONSISTENT WITH A 2-OSCILLATOR MODEL OF REGULATION

In many animals, changes in duration of nocturnal melatonin secretion chemically mediate effects of seasonal changes in nightlength on behav ior and physiology. According to one model, the changes in duration of secretion result from adjustments in the timing of two circadian osci llators, one entrained to dusk, controlling onset, and another entrain ed to dawn, controlling offset. Consistent with this model, in six wom en, we found separate and reproducible evening and morning peaks of me latonin secretion that might represent the separate expressions of rhy thms of two oscillators.