melatonin

noun

mel·​a·​to·​nin ˌme-lə-ˈtō-nən 

: 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

All light can affect melatonin production, but blue light suppresses it more effectively, according to Harvard Health Publishing. —Joshua Rapp Learn, Discover Magazine, 21 May 2025

Johns Hopkins research demonstrates blue light from screens disrupts melatonin production for 2-3 hours after exposure. —Charell G. Coleman, Forbes.com, 1 July 2025

The study found that eating two bananas daily increased circulating melatonin levels by more than 500%.7 Plus, the magnesium and potassium in bananas may also be associated with improved sleep.8 Still, more research is necessary to determine bananas’ role in sleep. —Amy Brownstein, Verywell Health, 25 June 2025

People spent the bulk of the day outside and didn’t receive any excess light once the sun set, allowing melatonin levels to rise and sleep to take hold. —Alex Tzelnic, Outside Online, 22 May 2025

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

Rhymes for melatonin

• calcitonin
• digitonin
• serotonin
• kronen
• onan
• phonon
• ronin
• santonin

Kids Definition

melatonin

noun

mel·​a·​to·​nin ˌmel-ə-ˈtō-nən 

: 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 

: 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