Melatonin (MLT) is the main hormone secreted by the pineal gland in the thalamus and is named for its ability to lighten the skin of frogs. From puberty onwards, there is a gradual increase in connective tissue in the human pineal gland and a constant deposition of salt, which means that its volume decreases, while the amount of melatonin synthesized and secreted decreases with age. 250 ng/L at ages 1–3 years and only 30 ng/L at ages 67–84 years. Despite the age difference, there is no significant difference in the daytime secretion rate. Melatonin secretion has an extremely typical “diurnal low and nocturnal high” cycle, with a peak at 2 am, synchronized with the daylight cycle. In women, melatonin fluctuations in the blood are synchronized with the menstrual cycle, with the highest levels on the eve of menstruation and the lowest during ovulation, with a peak-to-trough difference of up to 5 times. Melatonin has a wide range of effects on the nervous system, mainly sedative, hypnotic, analgesic, anticonvulsant and antidepressant effects. Melatonin action is negatively correlated with sex hormones and may play a counterbalancing role in gonadal development, sex hormone secretion and regulation of reproductive cycle activity. Melatonin is also involved in immune regulation and the adjustment of biological rhythms (e.g., the re-establishment of biological clock disruptions and the return of “jet lag”). It also affects cardiovascular, renal, pulmonary, gastrointestinal and other functions.