[Melatonin and its role the circadian function].

Melatonin is a principal hormone of the vertebrate pineal gland. Its production, being under control of a circadian oscillator, follows circadian rhythmicity with high values at night and low values during day-hours. As the nocturnal production of melatonin is proportional to the length of the night (or dark phase of an imposed light-dark illumination cycle), the hormone conveys an important chronobiological message to the whole body, playing the role of a biochemical clock and calendar. This article surveys a current knowledge on regulation of melatonin biosynthesis, receptors and function, and particularly provides a comprehensive view of the role the hormone may play in the vertebrate circadian system.