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[Melatonin and its role in human reproduction].

Melatonin may be a key factor in regulation of seasonal variation in gonadal activity. The circadian disturbances related to reproduction are probably subsequent to the seasonal change. Moreover, melatonin might also be considered essential to both spermatogenesis and folliculogenesis. Exposure to bright light, suppressing the concentration of melatonin in circulation, is hypothesized to be useful in treatment of both male and female infertility in couples with abnormal melatonin metabolism. In women, seasonal variation observed in the concentrations of gonadotropins and gonadal steroids is associated with melatonin synthesis. This hormone may exert an inhibitory effect on the GnRH pulse generator to decrease gonadotropin secretion. Consequently, it has been implicated in the control of the pubertal onset and timing of ovulation. The number of irregular menstrual cycles with anovulation in creases in winter compared to summer. In addition, there is circadian variation in the time of ovulation, occurring usually in the morning during summer and in the evening during winter. Hormonal interactions and phases of the cycle are important variables contributing to the fluctuations in melatonin levels during the menstrual cycle.