Secretion of melatonin is dictated by light.  It is believed that melatonin has two primary functions in humans—to help control your circadian (or biological) rhythm and regulate certain reproductive hormones.

Melatonin and the Circadian Rhythm

Your circadian rhythm is a 24-hour biological cycle characterized by sleep-wake patterns. Daylight and darkness help dictate your circadian rhythm. Light exposure stops the release of melatonin, and in turn, this helps control your circadian rhythms.

Melatonin secretion is low during the daylight hours and high during dark periods, which has some influence over your reaction to photoperiod (the length of day versus night). Naturally, photoperiod affects sleep patterns, but melatonin’s degree of impact over sleep patterns is disputed.

Melatonin and Reproduction

Melatonin blocks the secretion of gonadotropins (luteinizing hormone and follicle-stimulating hormone) from the anterior pituitary gland. These hormones are important to the proper development and functioning of the ovaries and testes.