MELATONIN - ROLE IN GATING NOCTURNAL RISE IN SLEEP PROPENSITY

The present article reviews the evidence that melatonin possesses slee p-inducing effects and that it gates the increase in nocturnal sleepin ess. It is shown that, without exception, all the studies that have in vestigated daytime administrations of melatonin reported increased sle epiness, even at doses that do not increase plasma levels of melatonin beyond its physiological levels. By contrast, nighttime increase in s leepiness was achieved only after administration of high doses. Based on these findings and on the precise coupling between the endogenous n octurnal increase in melatonin secretion and the opening of the sleep gate, it is suggested that melatonin participates in the regulation of the sleep-wake cycle by inhibiting the central nervous system wakeful ness generating system. This inhibition allows a smooth transition fro m wakefulness to sleep. Clinical findings on decreased levels of noctu rnal melatonin in chronic insomniacs, and on the efficacy of exogenous melatonin in improving sleep in melatonin-deficient insomniacs, are c ongruent with this hypothesis.