INTEGRATING THE ACTIONS OF MELATONIN ON HUMAN PHYSIOLOGY

Melatonin has a diverse range of physiological effects in humans. Repo rted effects include modulation of the sleep-wake, thermoregulatory, c ognitive, cardiovascular and immune systems. While integrating these b road-ranging effects is difficult when current paradigms are used, the diverse effects of melatonin on human physiology may be better unders tood by shifting our theoretical perspective. Traditionally, research has treated melatonin as a classical hormone for which a defined effec t in physiological systems and a mechanism of action can be elucidated . In this article, we suggest that it may be more appropriate to view melatonin as an evolutionally stable timing signal to which each speci es has adapted the timing of physiological processes. From this perspe ctive, it appears that the physiological role of melatonin in humans f alls into two categories. The first relates to the self-regulation of circadian timing by the suprachiasmatic nucleus-pineal complex. The se cond relates to the promotion of restorative or anabolic physiological processes. In humans, elevated melatonin levels have been associated with reduced core temperature, increased heat loss, decreased cardiova scular output, reduced alertness and enhanced immune responsiveness. T aken together, these changes suggest that melatonin may increase the p ropensity for physiological processes promoting nocturnal sleep or pro cesses that occur during the sleep period.