**Abstract**

Melatonin is a neurotransmitter released from the pineal gland. The presence of receptor sites in the hypothalamus, pituitary gland, ovaries, and testicles and secretion of pituitary hormones (FSH and LH) are some of the effects of this hormone on reproduction. In addition to its systemic effect, it also showed an effect on ovarian physiology with the detection of high levels in the follicular fluid and the presence of melatonin receptors in the ovarian cells. In addition, it has been determined that melatonin affects follicular growth, oocyte maturation, ovulation, and luteal function. It has been stated that the effects of melatonin on the male reproductive system are indirectly effective through the gonads and indirectly by affecting the hormones. Again, some studies have expressed that melatonin has strong antioxidant properties and affects reproductive physiology due to this effect. This section discusses the effect of melatonin on male and female reproductive physiology.

**Keywords:** melatonin, reproduction, physiology, FSH, LH, ovarium, testes, oxidative stress

### **1. Introduction**

Melatonin is a hormone secreted by the pineal gland and is known to be involved in the regulation of many body functions and the regulation of circadian and seasonal rhythms [1]. This hormone is rhythmically released from the epiphysis at night [2]. Since its release is regulated according to the nighttime, its secretion occurs with the internal reflection of the external photoperiod [3]. Melatonin is also called the "hormone of darkness". It has been determined that melatonin is synthesized in the lacrimal gland, retina, erythrocytes, platelets, and some cells in the gastrointestinal tract other than the pineal gland, but this synthesis has little effect on the plasma melatonin level [4–7]. The main synthesis site of melatonin is pinealocytes, and it is synthesized from tryptophan in these cells. Tryptophan is first converted to 5-hydroxytryptophan by the enzyme tryptophan hydroxylase. Then 5-hydroxytryptophan is converted to serotonin by amino acid decarboxylase. Serotonin is converted to N-acetyl serotonin by the enzyme N-acetyl transferase. N-acetyl serotonin is converted to melatonin by

the enzyme methyltransferase. Thus, the synthesized melatonin is released into the blood circulation [8, 9]. Around 19.00–20.00, melatonin secretion begins to increase, reaches its highest point between 02.00 and 04.00 at night, and decreases with the increase of daylight in the morning. It starts to decrease between 07.00 and 09.00 in the morning [10–12]. Melatonin takes part in many biological and physiological regulations in the body. It is effective on biorhythm (circadian rhythm) and also has direct and indirect effects on the reproductive system, regenerating our cells, regulating the immune system, anticarcinogen, antioxidant, antiaging, and reproductive system [13]. This chapter, it is aimed to explain the physiological effects of melatonin, which has important bodily effects in the body, on the female and male reproductive systems.
