melatonin

American  

[mel-uh-toh-nin] / ˌmɛl əˈtoʊ nɪn /

noun

Physiology.

1. 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.

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melatonin British  

/ ˌmɛləˈtəʊnɪn /

noun

1. the hormone-like secretion of the pineal gland, causing skin colour changes in some animals and thought to be involved in reproductive function

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melatonin Scientific  

/ mĕl′ə-tō′nĭn /

1. 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: C ₁₃ H ₁₆ N ₂ O ₂ .

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Closer Look

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.

Etymology

Origin of melatonin

Coined in 1958 by U.S. researcher Aaron B. Lerner and colleagues; from Greek mélā(s) “black” + tone + -in ²

Explanation

Melatonin is a hormone your body produces to help regulate your sleep cycle. Sometimes people take a melatonin supplement at night to help them get a better night's rest. You might think of melatonin as a nighttime hormone, and you'd be right — darkness actually stimulates the release of melatonin. Its name reflects this fact: the Greek root is melas, "black or dark." Scientists suspect that our modern lifestyle (electric lights, computer screens, and night shifts at work) can have harmful effects on the body's ability to produce and regulate melatonin, which not only makes us sleepy, but may protect against neurological conditions like Alzheimer's disease.

Example Sentences

A narrative review published in World Journal of Pediatrics, by researchers at Boston Children's Hospital explored the rapid rise of melatonin use among children and adolescents worldwide.

From Science Daily • Mar. 15, 2026

Testing of commercial melatonin supplements has revealed major differences between labeled doses and the actual amount of melatonin contained in some products.

From Science Daily • Mar. 15, 2026

These findings suggest that the risks associated with real world melatonin use may be higher than previously assumed.

From Science Daily • Mar. 15, 2026

Despite that perception, melatonin is actually a hormone that affects more than just sleep cycles.

From Science Daily • Mar. 15, 2026

“I can give you some melatonin to help you relax and fall asleep. You’ll be surprised how much a good night’s sleep can help you.”

From "Boy 2.0" by Tracey Baptiste