**2. Melatonin main physiological functions and its role in maintaining human health**

During the recent years, new data on the mechanisms providing for the integral interaction among the nervous, immune, and endocrine systems have been received. Presumably, pineal gland is an integrator of such interaction, while its main hormone, melatonin, takes part in regulation of the activity of central and vegetative nervous systems, endocrine organs, and immune system. The performed investigations have demonstrated that melatonin fulfills an extremely wide range of physiological functions: biorhythmic and immunomodulatory processes, thermal control and sleep onset, and antioxidative and anti-stress effects [3]. Hormone secretion starts on the third month of infant development and reaches its peak during the first years (not later than at the age of 5). Before puberty, melatonin synthesis remains at a constantly high level [4]. During the age of 11–14, due to the fact that the pineal gland reduces melatonin production, the hormone mechanisms of sexual development are launched. The next significant reduction in activity occurs simultaneously with menopause onset—at the age of 45–60. With the aging progression, along with decrease in basal level, melatonin secretion peaks are getting lower [1]. During daytime melatonin concentration in the blood serum remains low (10–20 pg/ml), while during the night hours it grows considerably (80,120 pg/ml) and reaches its maximum value between midnight and 3–5 a.m. Melatonin secretion usually starts at 9 p.m. and terminates at 7–9 a.m. Melatonin metabolites are found in urine: 6-sulfatoxymelatonin (80–90%) and 6-hydroxyglucuronide (10–20%) corresponding to the circadian rhythm that is very close to the rhythm of melatonin secretion [5].

A new science, biorhythmology, introduced the notion of desynchronosis clinically very important—that means ill-being or pathological syndrome, which is connected with the unbalance of circadian rhythms. A degree of desynchronosis is defined by the quantity and rhythm of melatonin production during the day and night. It has been determined that when a somatic disease goes hard or aggravates, melatonin production is getting worse, and its night indicator is getting closer to the day value [6]. Disturbed melatonin secretion finds its clinical manifestation in tiredness, indisposition, sleep disorder, and sometimes aggravation of a chronic disease or even appearance of a new one. Desynchronosis condition is exemplified by jet lag syndrome caused by rapid long-distance transmeridian travel [7].

It is generally known that melatonin has an antidepressant function. However, foreign colleagues stated disturbed circadian rhythm of melatonin secretion

experienced by patients with depression during a menopause along with its increase during the morning hours as compared with women in good health that also has impact on sleep, level of follicle-stimulating hormone, and body mass index [8]. A connection was established between sleep disorder and melatonin reduction in female saliva during perimenopause without registering such pattern among women in postmenopause [9].

Therefore, melatonin functions in a human body are quite diversified, and its normal secretion is highly important for maintaining human health in a contemporary world.
