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

According to Mack, foods that contain tryptophan, magnesium or compounds that support melatonin production may have a stronger connection to sleep. —Khloe Quill, FOXNews.com, 21 Dec. 2025

Blue light directly affects our sleep by suppressing the production of melatonin, the hormone that regulates our circadian rhythm. —Mélanie Nauche, Vogue, 15 Dec. 2025

When sunlight fades in the evening, the brain produces more of the hormone melatonin, and your core body temperature drops to promote sleep. —Jenalee Doom, The Conversation, 12 Dec. 2025

Drinking too much, or too late in the day, can suppress melatonin and disrupt sleep. —Lauren Panoff, Verywell Health, 9 Dec. 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