ABSTRACT
Over the past two decades, the pineal gland with its major hormone, melatonin, has been shown to control the seasonal changes in a wide variety of species. Nevertheless, its place in human physiology is unclear at present. Melatonin is a hormone secreted during the dark period of the day. On comparing different age groups, serum melatonin concentrations are at a peak during early childhood, decline steadily until puberty, remain stable and, finally, decrease in elderly people. Melatonin appears to have a physiologic role as a nocturnal sleep inducer in man, but it might also have a certain effect on the human circadian rhythms, the reproductive and the immune system. Patients suffering from pineal tumors, disturbed sexual maturation or a phase shift in their circadian rhythms may also show altered serum melatonin levels. Exogenous melatonin shows a direct sedative-hypnotic action, with no serious toxic or side effects even after ingestion of large doses. Melatonin may gain importance as a natural hypnotic substance in the near future.