Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A hormone derived from serotonin and produced by the pineal gland that stimulates color change in the epidermis of amphibians and reptiles and plays a role in sleep, aging, and reproduction in mammals.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A hormone, related to serotonin, that is secreted by the pineal gland, and stimulates colour change in the skin of reptiles, and is involved in the sleep/wake and reproductive cycles in mammals
  • n. Any material similar in its chemistry and effect to the natural hormone.

from the GNU version of the Collaborative International Dictionary of English

  • n. A hormone secreted by the pineal gland. Chemically it is N-acety-5-methoxytryptamine. Research has indicated that there are daily rhythms in secretion of melatonin, in particular due to the depressing effect on melatonin production by light received by the retina. Conversely it appears that melatonin may influence the circadian rhythms of animals. There is some experimental evidence that administration of melatonin may increase the amount of sleep in people with sleep disorders. However, the evidence is not convincing and the effect is not profound. Nevertheless, the deregulation of melatonin and its availability over-the-counter has led to many people taking melatonin to help sleep, without consulting a physician.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. hormone secreted by the pineal gland

Etymologies

Greek melās, black + ton(e) + -in.
(American Heritage® Dictionary of the English Language, Fourth Edition)

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