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

That’s why the Timeshifter jet lag app asks for your age, your usual sleep times, your caffeine and melatonin habits, and your chronotype—early bird, night owl, or neither. — Jd Shadel, Condé Nast Traveler, 22 Sep. 2026

Blood work showed significant increases in serotonin, melatonin and nitric oxide concentrations, and exercise testing showed gains in grip strength and reaction time. — Samantha Agate, Miami Herald, 22 Sep. 2026

Even a few lux of light can suppress melatonin, potentially shortening and fragmenting sleep, Wickramasinghe said. — Stephanie Anderson Witmer, HEALTH, 17 Sep. 2026

Light pollution from various sources in our bedroom can trick the brain into producing less melatonin, which can negatively impact sleep quality. — Melissa Rudy, FOXNews.com, 14 Sep. 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