Melatonin and human reproduction.

In photoperiodic nonhuman mammals the secretion of melatonin from the pineal gland plays a major role in regulating reproductive physiology; in humans these relationships are less clear. The melatonin rhythm changes throughout life with the first substantial change in nocturnal melatonin secretion being reportedly associated with puberty. The transition from Tanner stage 1 to Tanner stage 5 of sexual maturation is associated with a significant reduction in nocturnal melatonin levels, but a cause-effect relationship has not been established. Menstrual cyclicity has been reported to be associated with fluctuations in melatonin production but whether they are related to, eg ovulation or menstruation is not established. At high latitudes the quantity of melatonin produced by the pineal gland varies with season (changes in the light-dark cycle), and there is some evidence that this changes reproductive efficiency accordingly. Menopause is associated with a reduction in melatonin which may relate to the changing gonadotropin levels. In males of the same age melatonin levels also drop with no significant alteration in reproductive physiology. While correlations between melatonin and the status of the reproductive system in humans have been noted, whether they are functionally related remains to be determined.