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[mel-uh-toh-nin] /ˌmɛl əˈtoʊ nɪn/
noun, Physiology.
a hormone secreted by the pineal gland in inverse proportion to the amount of light received by the retina, important in the regulation of biorhythms: in amphibians, it causes a lightening of the skin.
Origin of melatonin
1955-60; < Greek mélā(s) black + tone + -in2 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for melatonin


the hormone-like secretion of the pineal gland, causing skin colour changes in some animals and thought to be involved in reproductive function
Word Origin
C20: probably from mela(nocyte) + (sero)tonin
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for melatonin

1958, from Greek melas "black, dark" (see melanin) + ending from serotonin. So called because its secretion is inhibited by sunlight.

Online Etymology Dictionary, © 2010 Douglas Harper
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melatonin in Medicine

melatonin mel·a·to·nin (měl'ə-tō'nĭn)
A hormone derived from serotonin and produced by the pineal gland that stimulates color change in the epidermis of amphibians and reptiles and that is believed to influence estrus in mammals.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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melatonin in Science
A hormone produced in the pineal gland that plays a role in regulating biological rhythms, including sleep and reproductive cycles. In many animals, melatonin also regulates the physiological effects that occur in response to seasonal changes, such as the growth of a winter coat of fur. Chemical formula: C13H16N2O2.

Our Living Language  : Melatonin, a natural hormone manufactured by the pineal gland in the brain, communicates information about light to different parts of the body. It helps regulate biological rhythms and plays an important role in the reproductive cycles of many animals. In humans it is best known for helping to regulate the body's circadian sleep-wake cycle. Melatonin production is affected by light exposure to the eyes; melatonin levels rise during the night and fall during the day, becoming almost undetectable. Though it does not actually induce sleep, melatonin can have sleep-promoting effects. Experiments have shown that at high doses melatonin lowers body temperature, decreases motor activity, and increases fatigue. Melatonin production starts falling after puberty, and it can virtually disappear in the elderly, a phenomenon which could help to explain why sleep disturbances are more prevalent among older adults. Marketed as a dietary supplement and touted as a cure-all for insomnia, jet lag, and even cancer and aging, the overall effects of melatonin on human health are still largely unknown.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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