The functions of the epiphyseal hormone melatonin are: 1)it is a marker of photo periodicity, especially the length of the interval from light to darkness; 2) it directs the physiological and endocrinological processes that influence the interval from light to darkness; and 3) seasonal variations are the basis of such processes. The pineal gland in mammals is connected to the network of nerves that activates or inhibits the enzyme system of the synthesis of melatonin. Darkness stimulates it and light inhibits it. In humans the concentration of circulating melatonin during the day is less than 20 pg/ml, but at night it rises 4- or 5-fold. However, the plasma melatonin level fluctuates depending on the phase of menstruation: the peak is at the beginning of the cycle, and it decreases to the minimum at around ovulation (the maximum is about 4.5 times higher than the minimum). The association between melatonin and ovulation is the hypothalamic generator of pulsatile secretion of the gonadotropin releasing hormone (GnRH). In golden hamsters melatonin lowered the expression of estrogen receptors in the neurons of the hypothalamic area, preoptica medialis. In women with hypothalamic amenorrhea and positive progesterone test as well as in ovulating women the nighttime secretion of melatonin peaked at about 3 o'clock in the morning, but the level of plasma melatonin was significantly higher at night and during the day than in healthy women. It seems that there is a connection between the beginning of preovulatory appearance of luteinizing hormone (LH) and early morning drop in melatonin level in the sense that melatonin contributes to its initiation. There have been good results in recent research to inhibit the generator of pulsatile secretion of GnRH necessary for ovulation by using melatonin in a new type of oral contraceptive. The oral contraceptive uses melatonin rather than estrogen, either alone or in combination with norethindrone.

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