melatonin

noun

mel·​a·​to·​nin ˌme-lə-ˈtō-nən How to pronounce melatonin (audio)
: a vertebrate hormone that is derived from serotonin, is secreted by the pineal gland especially in response to darkness, and has been linked to the regulation of circadian rhythms

Examples of melatonin in a Sentence

Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Waking at a consistent time, especially with morning light, helps keep that clock aligned with the day and regulate the timing of melatonin release at night. — Emily Cegielski, Flow Space, 28 Oct. 2026 Those affected could benefit instead from treatments, such as melatonin or light therapy, that specifically target the body’s circadian rhythms . — Harriet Marsden, TheWeek, 28 Oct. 2026 Treatments that target the body’s internal clock, such as melatonin or light therapy, may prove beneficial. — Payal Dhar, Scientific American, 27 Oct. 2026 Ask your doctor if melatonin or magnesium supplements are appropriate for you. — Ricardo Correa, EverydayHealth.com, 27 Oct. 2026 See All Example Sentences for melatonin

Word History

Etymology

mela-, extracted from Greek melan-, mélās "black, dark" + -tonin (in serotonin); so named because the hormone can lighten skin color in animals by reversing the effect of melanocyte-stimulating hormones — more at melano-

Note: The word was probably introduced in the first published description of the hormone's isolation: Aaron B. Lerner, et al., "Isolation of melatonin, the pineal gland factor that lightens melanocytes," Journal of the American Chemical Society, vol. 80, no. 10 (May 20, 1958), p. 2587.

First Known Use

1958, in the meaning defined above

Time Traveler
The first known use of melatonin was in 1958

Rhymes for melatonin

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Browse Nearby Words

melastome
melatonin
melatope
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Cite this Entry

“Melatonin.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/melatonin. Accessed 1 Nov. 2026.

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Kids Definition

melatonin

noun
mel·​a·​to·​nin ˌmel-ə-ˈtō-nən How to pronounce melatonin (audio)
: a hormone that is secreted by the pineal gland especially in response to darkness and has been linked to the regulation of bodily functions or activities (as sleep) that occur in approximately 24 hour cycles

Medical Definition

melatonin

noun
mel·​a·​to·​nin ˌmel-ə-ˈtō-nən How to pronounce melatonin (audio)
: a vertebrate hormone C13H16N2O2 that is derived from serotonin, is secreted by the pineal gland especially in response to darkness, and has been linked to the regulation of circadian rhythms

More from Merriam-Webster on melatonin

Britannica.com: Encyclopedia article about melatonin

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