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

Milk contains tryptophan and calcium, which help the body make sleep-regulating hormones like serotonin and melatonin. —Lindsey Desoto, Health, 28 Nov. 2025

That means eating turkey doesn't increase brain tryptophan levels enough to boost serotonin or melatonin, which are the hormones that help regulate sleep. —Khloe Quill, FOXNews.com, 27 Nov. 2025

One line of research looks at whether sour cherries or beefsteak tomatoes might be useful as a sleep aid, because these foods are known to be rich in naturally occurring melatonin. —Daniel Engber, The Atlantic, 26 Nov. 2025

From the melatonin in tart cherries to the magnesium in pumpkin seeds, the best foods for sleep offer natural ways to support relaxation and better sleep. —Lauren Manaker, SELF, 26 Nov. 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