definition about melatonin
Britannica
hormone secreted by the pineal gland, a tiny endocrine gland situated at the centre of the brain. Melatonin was discovered in 1958 by American physician Aaron B. Lerner and his colleagues at Yale University School of Medicine. Melatonin, a derivative of the amino acid tryptophan, is produced in humans, other mammals, birds, reptiles, and amphibians.
In humans, melatonin seems to play an important role in the regulation of sleep cycles. The pineal gland\'s production of melatonin varies both with the time of day and with age; production of melatonin is dramatically increased during the nighttime hours and falls off during the day, and melatonin levels at night are higher in children than in adults. It is thought that the increased production of melatonin coincident with nightfall acts as a fundamental mechanism for making people sleepy. With dawn the pineal gland stops producing melatonin, resulting in wakefulness and alertness. The high level of melatonin production in young children may explain their tendency to sleep longer than adults.
In mammals other than humans melatonin also possibly acts as a breeding and mating cue, since it is produced in greater amounts in response to the longer nights of winter and less so during summer. Animals who time their mating or breeding to coincide with favourable seasons (such as spring) may depend on melatonin production as a kind of biological clock that regulates their reproductive cycles on the basis of the length of the solar day. In amphibians, melatonin stimulates a lightening of the skin.