Melatonin is an indole hormone that is produced by the pineal gland, mainly at night, with a peak around 3.00 a.m. under normal environmental conditions. This endogenic secretion cycle is generated by the suprachiasmatic nuclei in response to the day/night alternation. Light either suppresses or entrains melatonin production according to the time of light exposure. Melatonin can be viewed as the "hand" of the internal clock and is regulated via the central nervous and sympathetic systems. Melatonin synchronizes biological cycles, particularly the temperature and sleep/wake cycles. Exogenous melatonin can influence the endogenous secretion of melatonin according to a phase response curve, an effect that provides a rationale for the use of melatonin to treat disorders of biological rhythms (rapid time-zone change syndrome, delayed sleep phase syndrome, desynchronization in blind subjects or shift workers, insomnia in the elderly).