Melatonin is a hormone produced by the human body in response to the light conditions of the surrounding environment. It acts as a messenger of a complex circuit in the brain, to which the regulation of the sleep-wake rhythm during the day is subject. Fluctuations in the release of melatonin are the result of external influences or altered metabolic processes and can have a decisive influence on sleep quality.
What is melatonin?
Schematic diagram showing the anatomy and structure of the endocrine system (hormone system). Click to enlarge. Chronobiology is concerned with the study of functions that living organisms organize along the circadian (around the day) rhythm of recurring daily cycles. Living organisms control processes that conform to circadian rhythms via specific metabolic processes. These metabolic processes are themselves subject to very different influences. External environmental conditions, living habits, age, and the individual functioning of an organism are factors that affect the regulation of time-controlled biological processes. For humans, the sleep-wake rhythm is one of the most important circadian cycles. It depends on the production of melatonin in the body. Melatonin is a hormone that the organism can synthesize itself. It acts in the function of a neurotransmitter. Neurotransmitters are biochemical messengers responsible for the transmission of stimuli between nerve cells and other cells. For the production of melatonin, the alternation of light and dark is the crucial external zeitgeber.
Production, formation, and manufacture
The main site of production of the hormone melatonin in the human body is the pineal (or epiphysis) gland. It belongs to the structures of the epithalamus, which is itself a part of the diencephalon. Only melatonin from the pineal gland has a sleep-promoting effect, but the intestine and retina of the eye can also synthesize the hormone. The production of melatonin is the result of functions of a complicated circuit. For this purpose, the pineal gland is connected to the hypothalamus, which is also part of the diencephalon. Light-dark stimuli received through the retina are transmitted to the hypothalamus and from there reach the pineal gland. Under the influence of light, the pineal gland inhibits the synthesis of melatonin from the parent substance serotonin. Only in darkness does production increase. The secretion of melatonin at night is three times higher in older people and up to twelve times higher in younger people than during the day. Peak secretion is reached around 03:00 in the morning, although the timing varies somewhat with the seasons.
Function, effects, and properties
Melatonin has a sleep-inducing effect. Because its secretion is inhibited by daylight and it is produced predominantly in the dark, the natural phase of human sleep is at night. Deep sleep, which is promoted by melatonin, is a crucial period for important metabolic processes to take place. While the release of the growth hormone somatropin, which has a positive influence on the strengthening of tendons, connective tissue and muscles, is stimulated during this phase, melatonin has a rather curbing effect on many other bodily functions. This applies to the regulation of kidney function and blood pressure, to the modulating influence on general stress reactions and to the functions of the sex glands. Melatonin also helps to bind free radicals and prevents cell destruction with its antioxidant effect. Studies show that melatonin can bind to white blood cells and thus have a positive influence on the immune system. Melatonin can be supplied to the body from outside. In Germany, it is available on prescription only. In a dosage of 2 mg, it is approved for patients over 55 years of age for the treatment of primary sleep disorders. The bioavailability of the ingested melatonin is 15%. Ingestion should be of short duration because long-term effects have not been adequately studied.
Diseases, ailments, and disorders
Since the circadian sequence of the secretion of melatonin is subject to the alternation of light and dark as a timer, fluctuations in the day-night rhythm directly affect the supply of the hormone.Environmental conditions such as the long nights and short days of our wintertime and phenomena such as polar day and polar night can affect the chronobiological rhythm and interfere with the regulated periodicity of the release of melatonin. Our modern way of life, which imposes night work and intercontinental flights with rapid changes of time zones, also disturbs the sequence of the clock-giving alternation between day and night. Ultimately, however, our biological aging process is also responsible for the fact that melatonin secretion decreases with time and a deficit of availability arises in the organism. Regardless of its cause, both curbed and increased production of melatonin can be associated with sleep disturbances and a general imbalance in the alternation of rest and activity phases. While generally rejuvenating effects or even a curative ability in cancer for the addition of melatonin are not scientifically supported, a short-term treatment helps to intercept the consequences of deficiency symptoms and to alleviate sleep disturbances in special life situations.
