MELATONIN IN HUMANS - A BIOCHEMICAL MARKER OF THE CIRCADIAN CLOCK ANDAN ENDOGENOUS SYNCHRONIZER

Melatonin (MLT) is a methoxyindole secreted principally by the pineal gland. It is synthesized at night under normal environmental condition s. The endogenous rhythm of secretion is generated by the suprachiasma tic nuclei and activated by the light/dark cycle. Light is able to bot h suppress or activate melatonin production on the light schedule. The nycthohemeral rhythm of this hormone can be determined by repeated me asurements of plasma or saliva MLT or urine sulfatoxy-MLT, the main he patic metabolite. Melatonin can be considered as the output (the hand) of the endogenous clock. Since the regulating system follows a centra l and sympathetic nervous pathway, an abnormality at any level could u nspecifically modify the MLT secretion, especially in patients with sy mpathalgia or dysautonomia. Melatonin plays the role of an endogenous zeitgeber on core temperature or sleep-wake cycle. Exogenous MLT is ab le to influence the endogenous secretion of the hormone according to a phase response curve. There are practical implications for this prope rty in situations when biological rhythms are disturbed (jet-lag syndr ome, delayed sleep phase syndrome, insomnia in blind people, shift-wor k insomnia in elderly people). Improvement of pharmaceutical forms (co ntrolled release preparations) or development of MLT analogs could lea d to decisive progress.