A hormone produced by the pineal gland, melatonin is intimately involved in regulating the sleeping and waking cycles, among other processes. Melatonin supplements are sometimes used by people who have chronic insomnia. Always see your doctor before taking melatonin, as it is not recommended for all patients with sleep problems.
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N -Acetyl-5-methoxytryptamine; a substance formed by the mammalian pineal gland, which appears to depress gonadal function in mammals and causes contraction of amphibian melanophores; a precursor is serotonin; m. is rapidly metabolized and is taken up by all tissues; it is involved in circadian rhythms. [melanophore + G. tonos, contraction, + -in] M. secretion is linked to both the sleep-wakefulness and light-dark cycles. Ocular perception that ambient light is dimming has been shown to trigger, via neural pathways involving the hypothalamus, increased secretion of m. by the pineal gland. Serum levels increase 10-fold just before sleep and peak around midnight. Twenty-four-hour secretion is higher in winter than in summer. The decline of m. secretion with age has been blamed for the tendency to insomnia in the elderly. Because m. acts as an antioxidant in counteracting free radicals, it has been promoted as a means of delaying aging and preventing cancer, heart disease, and Alzheimer dementia. It has also been proposed as an antidepressant because serotonin (5-hydroxytryptamine), whose metabolism is known to be disordered in clinical depression, is a chemical precursor of m.. Adequately controlled, large-scale studies of the efficacy, safety, and optimum dosage of m. are lacking. There is experimental evidence that long-term administration can reset the circadian pacemaker. Anecdotal reports suggest that shorter courses can hasten recovery from jet lag and facilitate adaptation to night-shift work. In one controlled study of 15 emergency physicians, m. did not improve sleep when subjects returned to a normal sleep pattern after working night shifts. The direct soporific effect of m. varies widely from person to person. Limited studies suggest that it may increase the duration of restful nighttime sleep in the elderly. High doses of m. result in prolonged elevation of serum m. level and increased production of prolactin by the pituitary gland. Unlike most hormones, m. is readily absorbed from the digestive tract and is a component of some foods. Hence therapeutic formulations are not subject to federal drug regulations or purity standards. Testing of commercially available preparations of m. has indicated both variation in potency and the presence of possibly harmful contaminants.

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mel·a·to·nin .mel-ə-'tō-nən n a vertebrate hormone C13H16N2O2 that is derived from serotonin, is secreted by the pineal gland esp. in response to darkness, and has been linked to the regulation of circadian rhythms

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a hormone produced by the pineal gland in darkness but not in bright light. Melatonin receptors in the brain, in a nucleus immediately above the optic chiasma, react to this hormone and synchronize the nucleus to the 24-hour day/night rhythm, thus informing the brain when it is day and when it is night. Melatonin is a derivative of serotonin, with which it works to regulate the sleep cycle, and is being used experimentally to treat jet lag, SADS, and insomnia in shift workers and the elderly.

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mel·a·to·nin (mel″ə-toґnin) a hormone synthesized by the pineal gland in many species of animals; its secretion increases during exposure to light. In adult amphibians it produces lightening of the dermal pigmentation by promoting aggregation of melanosomes. In mammals it influences hormone production and in many species it regulates seasonal changes such as reproductive pattern and fur color. In humans it is implicated in the regulation of sleep, mood, puberty, and ovarian cycles. It has been tried therapeutically for a number of conditions, including insomnia and jet lag.

Medical dictionary. 2011.

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