[ABSTRACT] Melatonin is synthesized and secreted during the dark period of the light/dark cycle. The rhythmic nocturnal melatonin secretion is directly generated by the circadian clock, located in mammals within the suprachiasmatic nuclei (SCN), and is entrained to a 24-hour period by the light-dark cycle. The periodic secretion of melatonin may be used as a circadian mediator to any system than can read the message. In addition, direct effects of the hormone on the SCN could explain some of m