MELATONIN

the only hormone secreted by the pineal gland. (The pineal gland is a tiny endocrine gland situated at the centre of the brain.) Melatonin was discovered in 1958 by Aaron B. Lerner and other researchers working at Yale University. Melatonin is produced in humans, other mammals, birds, reptiles, and amphibians. It is present in very small amounts in the human body. Melatonin was previously known to cause the skins of amphibians to blanch, but its functions in mammals remained uncertain until research discoveries in the 1970s and '80s suggested that it regulates both sleeping cycles and the hormonal changes that usher in sexual maturity during adolescence. 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 are much higher in children under age seven than in adolescents and are lower still in adults. Melatonin apparently acts to keep a child's body from undergoing sexual maturation, since sex hormones such as luteotropin, which play a role in the development of sexual organs, emerge only after melatonin levels have declined. This hypothesis is supported by the fact that children with tumours of the pineal gland often reach sexual maturity unusually early in life, presumably because the pineal's production of melatonin has been hampered. Melatonin also seems to play an important role in regulating sleeping cycles; test subjects injected with the hormone become sleepy, suggesting 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, and wakefulness and alertness ensue. The high level of melatonin production in young children may explain their tendency to sleep longer than adults. In mammals other than humans melatonin 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.