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an indoleamine hormone synthesized and released by the pineal body during the hours of darkness; it may have a role in the control of the regulation of gonadotropin release.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


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. Melatonin is rapidly metabolized and is taken up by all tissues. It is involved in circadian rhythms.
[melanophore + G. tonos, contraction, + -in]

Melatonin secretion is linked to both sleep-wakefulness and light-dark cycles. Ocular perception that ambient light is dimming has been shown to promote increased secretion of melatonin by the pineal gland through neural pathways involving the hypothalamus. Serum levels increase tenfold just before sleep and peak around midnight. Twenty-four-hour secretion is higher in winter than in summer. The decline of melatonin secretion with age has been blamed for the tendency to insomnia in the elderly. Because melatonin 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), the metabolism of which is known to be disordered in clinical depression, is a chemical precursor of melatonin. Adequately controlled, large-scale studies of the efficacy, safety, and optimal dosage of melatonin are lacking. There is experimental evidence that long-term administration can reset the circadian pacemaker. Several studies support the effectiveness of melatonin in preventing or reducing jet lag, particularly for travelers flying east across five or more time zones. The direct soporific effect of melatonin 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 melatonin result in prolonged elevation of serum melatonin level and increased production of prolactin by the pituitary gland. Unlike most hormones, melatonin 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 melatonin has indicated both variation in potency and the presence of possibly harmful contaminants.

Farlex Partner Medical Dictionary © Farlex 2012


An animal hormone, C13H16N2O2, derived from serotonin and produced by the pineal gland. It stimulates color change in the skin of amphibians and reptiles and plays a role in regulating circadian rhythms and reproductive cycles in mammals. It is also found in plants and fungi.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


A hormone formed by methylation and acetylation of serotonin, which is produced in a diurnal cycle by the pineal gland in response to light. L-tryptophan is metabolised to 5-hydroxytryptophan which in turn is metabolised to serotonin and finally to N-acetyl-serotonin (melatonin). The pineal gland is similar to the photosensory organ of lower vertebrates; its binding sites are concentrated in the suprarhiasmatic nucleus of the hypothalamus (directly connected to the eyes) and possibly also to the biological clock. Melatonin secretion is intimately linked to the light-dark cycle, which peaks around midnight, and is highest in winter; it decreases with age. Production may be markedly increased in patients with hypothalamic or hypogonadotropic hypogonadism, or delayed puberty, and may be decreased in precocious puberty. It helps sleep, stimulates the immune system, may reverse ageing phenomena and may be of use in treating depression.

Melatonin should not be used in those with leukaemia, Hodgkin lymphoma or myeloma; it should not be used in pregnancy.
Alternative medicine
Melatonin has acquired currency in the alternative healthcare community as a self-administered tonic; some believe melatonin can be used to slow the ageing clock, extend a person’s lifespan, invigorate one’s sex life, slow the growth of cancer cells, treat insomnia, prevent heart disease and revive the immune system.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


A substance formed by the pineal gland that appears to depress gonadal function; serotonin is a precursor; melatonin is rapidly metabolized and is taken up by all tissues; it is involved in circadian rhythms.
Compare: bioregulator
[melanophore + G. tonos, contraction, + -in]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


A hormone synthesized from serotonin in the pineal gland and elsewhere. Melatonin production has a strong circadian rhythm, being secreted mainly in the period between about 2100 hours and 0800 hours. Bright light suppresses melatonin secretion and exogenous melatonin can alter the timing of the body clock. For these reasons it has been proposed as a means of combatting jet lag. Other methods have been found more generally useful.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005


a hormone of the pituitary gland thought to inhibit reproductive activities.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


A naturally occurring hormone involved in regulating the body's "internal clock."
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


An opsin-like protein, sensitive to light with a peak sensitivity around 480 nm, and found in the very small proportion of retinal ganglion cells which are photosensitive. It is believed to be the visual pigment that synchronizes the circadian cycle to the day-night cycle as well as being involved in the control of pupil size and the release of melatonin. This neural circuit appears to be independent of the conventional retinal phototransduction in the rods and cones. See pupil light reflex.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann


Substance formed by mammalian pineal body, which appears to depress gonadal function in mammals; a precursor is serotonin.
[melanophore + G. tonos, contraction, + -in]
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about melatonin

Q. is there a blood test for melatonin

A. Technically there is but it is not a simple blood test and will not be done on a regular basis, and is probably very expenisve also.

Q. Any other treatment for sleeplessness problem than Melatonin??? 33 years male having acute sleeping problem awake until 08:00-09:00am. Taking Melatonin failed.

A. Belladonna. [Bell]
The sleepless conditions calling for Belladonna are due to congestion; sleep is extremely restless, as a rule it is interrupted by talking, startings, muscular jerkings and spasmodic motions; frightful images appear on closing the eyes and the patient therefore dreads sleep. Children awake from sleep frightened. The dreams found under Belladonna are frightful ones, and they constantly awaken the patient. It is probably our best remedy for insomnia due to cerebral hyperaemia; that is, it will be most often indicated, also after morphine which produces cerebral hyperaemia of a passive variety. Aconite comes in here, too, but with Aconite there is intense anxiety and restlessness, fear of disaster or death. Cuprum, Stramonium and Zincum have the symptom that the patient is aroused from sleep frightened. For the complete list: Hope this helps.

More discussions about melatonin
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