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

This dreamy blend includes 3 grams of melatonin along with L-theanine, chamomile and lemon balm. —Bestreviews, Chicago Tribune, 8 May 2026

How Circadian Lighting Works Specialized cells in the retina are tuned to short-wavelength blue light around 480 nanometers and send signals straight to the master clock, telling it when to release melatonin and when to suppress it. —Allison Palmer, Kansas City Star, 7 May 2026

Morning sun cues the body to ramp up alertness and wind down melatonin in the evening, setting a rhythm that supports steadier energy through the day and easier sleep at night. —Samantha Agate, Fort Worth Star-Telegram, 6 May 2026

Take melatonin a few hours after the last meal of the day. —Ayesha Gulzar, Verywell Health, 5 May 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