Influences of melatonin on human circadian rhythms

Administration of melatonin is useful in the treatment of desynchronized conditions. The mechanisms through which melatonin exerts its effect are not completely clear. Melatonin exerts direct effects on several biological functions, such as the regulation of body temperature, but there is no proof that these actions are important in the indirect regulation of main pacemaker activity. By contrast, it is very likely that melatonin exerts direct effects on circadian clocks, and that depending on the time of its administration/presence, it antagonizes or promotes the phase-shifting effects exerted by light. It is possible that melatonin regulates its own secretion and that its prolonged or shortened secretion in the period of the night-day transition is responsible for the lengthening or shortening, respectively, of the nocturnal melatonin rise. This possibility that needs to be confirmed by extensive studies may represent a physiological mechanism through which photoperiodic information is more rapidly and efficiently transformed by melatonin in a circadian signal to all the body (Chronobiology International, 14(2), 205-220, 1997).