Bimodal patterns of human melatonin secretion consistent with a two-oscillator model of regulation

In many animals, changes in duration of nocturnal melatonin secretion chemically mediate effects of seasonal changes in nightlength on behavior and physiology. According to one model, the changes in duration of secretion result from adjustments in the timing of two circadian oscillators, one entrained to dusk, controlling onset, and another entrained to dawn, controlling offset. Consistent with this model, in six women, we found separate and reproducible evening and morning peaks of melatonin secretion that might represent the separate expressions of rhythms of two oscillators.