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

Warm milk delivers tryptophan, the amino acid your body converts into serotonin and melatonin. — Lauren Jarvis-Gibson july 25, Miami Herald, 25 July 2026

The Oura Ring education content emphasizes that first-thing sunlight suppresses melatonin and boosts cortisol. — Hanna Wickes, Charlotte Observer, 25 July 2026

The Oura Ring emphasizes morning sunlight in its education content, telling users that first-thing sunlight suppresses melatonin and increases cortisol, and positioning the ring as a way to validate the impact of sunmaxxing. — Hanna Wickes, Sacbee.com, 25 July 2026

Warm milk delivers tryptophan, an amino acid the body uses to produce serotonin and melatonin, which is why the old-school bedtime drink has stuck around. — Lauren Jarvis-Gibson july 24, Fort Worth Star-Telegram, 24 July 2026

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