Melatonin phase advances circadian rhythm.

We studied the effect of acute (1 day) and subacute (7 days) treatment with melatonin (0.5 mg) on the endogenous rhythms of melatonin secretion in 12 healthy male volunteers, using a placebo-controlled, double-blind, cross-over design. Melatonin given at 1700 h for 7 days significantly advanced the onset of endogenous melatonin secretion, while a single dose was without effect. These data are consistent with the hypothesis that melatonin plays a role in the organisation of circadian rhythms in humans and suggest that appropriately timed melatonin administration may provide a means of altering the timing of circadian cycles.