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

Quality drops around menstruation as estrogen and progesterone fall, blunting melatonin and cortisol rhythms. —Allison Palmer updated June 13, Sacbee.com, 13 June 2026

Why Sleep Changes Across Your Cycle Sleep quality tends to drop around menstruation as falling estrogen and progesterone blunt melatonin and cortisol rhythms. —Allison Palmer updated June 13, Fort Worth Star-Telegram, 13 June 2026

This wooden nightstand has just enough drawer space for stashing nightly reads, melatonin, and your favorite sleep mask. —Julia Harrison, Architectural Digest, 12 June 2026

Natural light helps regulate your circadian rhythm—aka, your body’s internal clock that governs sleep-wake cycles and the production of hormones like melatonin. —Allison Forsyth, Health, 9 June 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