[Melatonin and biological rhythms: various aspects in human physiopathology]

Melatonin (N-Acetyl-5-methoxytryptamine) is a hormone secreted mainly by the pineal gland or epiphyse and in smaller amounts by the retina. It is biosynthesized from tryptophan, the two critical enzymatic steps depend upon N-Acetyl-transferase (NAT) and 5-hydroxyindole-O-methyltransferase (5-HIOMT). The circadian rhythm of melatonin is the same in man and all the laboratory animals studied until now with noctural plasma concentrations 3-10 times greater than during daytime. The secretion and release of melatonin depend upon a large number of exogenous and endogenous factors as e.g. sex, age, pubertal stage, menstrual cycle, drugs, season... Light is the major regulating factor which acts through the retino-hypothalamic tract. Melatonin is considered as a transducer of the light signal forwarding to the organism the information about day length (relative length of day and night). It is a time-clue provider used by the organism to adapt itself to its environment.