Prior to three months of age there is little melatonin (MLT) secretion in humans. MLT production then commences, becomes circadian, and reaches its highest nocturnal blood levels between the ages of one to three years. During the remainder of childhood, nocturnal peak levels drop progressively by 80%. In adults, these levels show an additional drop of some 10%, mainly during senescence. The large drop in serum MLT during childhood is probably the result of the increase in size of the human body, despite a constant MLT production after infancy. The additional decline of MLT with higher age may be due to a yet unidentified physiological mechanism accompanying senescence. The biological significance of these MLT alterations remains unknown. Since the discovery of MLT, an immediate sedative action of this hormone has been known. A number of recent studies have demonstrated that MLT indeed exerts a sleep-promoting action by accelerating sleep initiation, improving sleep maintenance, and marginally altering sleep architecture. The potential of MLT in the treatment of insomnia is being explored, and the results are promising. Although in most of these studies pharmacological dosages of MLT have been used, preliminary data suggest that similar effects can also be achieved by physiological hormone concentrations. The latter observation raises the question of whether MLT might be involved in the physiological control of sleep.