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Longman Dictionary of Contemporary English
melatonin
noun
EXAMPLES FROM CORPUS
▪ A synthetic form of melatonin can be bought over-the-counter at health food stores and many pharmacies.
▪ Daily injections of melatonin given at 3.30 each afternoon, entrained the rats to a 24 hour cycle.
▪ Perhaps melatonin would help him get a little rest.
▪ Starlings, which are active during the day, lock on to melatonin when it coincides with the end of their activity.
▪ The rat does not immediately adjust its daily rhythm to the melatonin.
▪ The results raise an interesting problem for people studying other effects of melatonin.
▪ There may be uncertainties surrounding melatonin, but it is clearly the dose of the moment.
▪ When the sun goes down, the eyes cue the gland to start pumping melatonin.
The Collaborative International Dictionary
melatonin

melatonin \mel`a*tonin\ n. (Physiology) 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.

A role for melatonin in sleep facilitation has been inferred from its effect on electroencephalogram patterns, but it has not been possible to demonstrate that wakefulness sleep cycles are driven by periodic accumulation, depletion, or regeneration of melatonin.
--Uwe Ackermann, Essentials of Human Physiology, 1992

Douglas Harper's Etymology Dictionary
melatonin

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

Wiktionary
melatonin

n. 1 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 2 Any material similar in its chemistry and effect to the natural hormone.

WordNet
melatonin

n. hormone secreted by the pineal gland

Wikipedia
Melatonin

Melatonin, chemically N- acetyl-5- methoxy tryptamine, is a substance found in animals, plants, fungi, and bacteria. In animals, it is a hormone that anticipates the daily onset of darkness; however in other organisms, it may have different functions. Likewise, the synthesis of melatonin in animals differs from that in other organisms.

In animals, melatonin is involved in the entrainment (synchronization) of the circadian rhythms of physiological functions including sleep timing, blood pressure regulation, seasonal reproduction, and many others. Many of melatonin's biological effects in animals are produced through activation of melatonin receptors, while others are due to its role as a pervasive and powerful antioxidant, with a particular role in the protection of nuclear and mitochondrial DNA.

It is used as a medication for insomnia, however, scientific evidence is insufficient to demonstrate a benefit in this area. Melatonin is sold over-the-counter in the United States and Canada. In other countries, it may require a prescription or it may be unavailable.

Melatonin (album)

Melatonin is Jonathan Seet's first album, released independently in 2000.

Usage examples of "melatonin".

I was ordered three times to fly nonstop from Tokyo to New York, a flight of about the same duration, but because I was crossing ten time zones, my pineal gland secreted melatonin so abnormally that it required me four to five days to bring it back into balance.

Estrogens and progesterone are just part of a complex and closely interlinked hormonal system that includes many other hormones, including testosterone, DHEA, pregnenolone, and melatonin, to name just a few.

The treatment now included supplements of the enzyme photolyase, to correct DNA damage, and supplements of the pineal hormone melatonin, and dehydroepiandrosterone, a steroid hormone produced by the adrenal glands.

The air is thick with the smells of tobacco, brewer's yeast, and melatonin spray: Half the dotters are nursing monster jet lag hangovers, and the other half are babbling a Eurotrash creole at each other while they work on the hangover.

Thus at the end of the day he would sit down apprehensively or hopefully, and listen to what the AI had captured during the day: and mostly it was thought that he remembered thinking, but occasionally he would hear himself say, “Synthetic melatonins may be a better antioxidant than natural ones, so that there aren’t enough free radicals,” or “Viriditas is a fundamental mystery, there will never be a grand unified theory,” without having any memory of saying such things, or, often, what they might mean.

He reaches for the bottle beside the bed, dry-swallows two melatonin tablets, a capsule full of antioxidants, and a multivitamin bullet: Then he lies down on the bed, on his back, legs together, arms slightly spread.

He reaches for the bottle beside the bed, dry-swallows two melatonin tablets, a capsule full of antioxidants, and a multivitamin bullet: then he lies down on the bed, on his back, legs together, arms slightly spread.

All the way up to the large organismic issues, such as brain-wave rhythms and their relationship to the heart and other organs, or the pineal gland's ever-decreasing secretions of melatonin, a hormone that seemed to regulate many aspects of aging.

Wallet leaking credit cards and carelessly wadded bills, liquid makeup in small jars, loose-powder compact, dented lipsticks, of prescription pills, multiple vitamins, calcium and melatonin, book and cellular phone and a half-dozen felt-tip pens, and--what was this?