cir·​ca·​di·​an | \ sər-ˈkā-dē-ən How to pronounce circadian (audio) \

Definition of circadian

: being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function) circadian rhythms in activity

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Did You Know?

Just over fifty years ago, no one talked about "circadian rhythms" - because "circadian" hadn't even been coined yet. In 1959, a scientist formed the word from the Latin words circa ("about") and "dies" ("day"), and it caught on quickly. "Circadian" appeared in periodicals throughout the sixties, and appeared in a Merriam-Webster dictionary before the decade was up. Most often, it's seen and heard in the term "circadian rhythm," which refers to the inherent cycle of about 24 hours that appears to control various biological processes, such as sleep, wakefulness, and digestive activity. If you want to impress your friends, you can also use the term "circadian dysrhythmia," a fancy synonym of "jet lag."

Examples of circadian in a Sentence

Recent Examples on the Web Physical activity and exposure to natural light are important to maintaining a normal circadian rhythm. Julie Jargon, WSJ, "Screens Before Bed Are OK for Kids—Just Keep It Calm," 20 Oct. 2020 The circadian-rhythm color lights turn a cream bathroom temporarily pink. House Beautiful, "This Stunning Bel Air Estate Is One of a Kind (and Built for Future Parties)," 18 Sep. 2020 Blue light from devices can interrupt our normal circadian rhythms and throw off sleep and wake schedules. Gulnaz Khan, National Geographic, "How to keep your kid digitally healthy," 14 Sep. 2020 Melatonin, while a popular sleep aid, influences our circadian rhythm that regulates REM sleep, and can lead to more or fewer nightmares. Kristen Rogers, CNN, "10 ways to conquer adult nightmares and get better sleep," 4 Sep. 2020 But when magnetic fields were also blocked, the volunteers’ circadian cycles deteriorated further. Jo Marchant, Wired, "The Oysters That Knew What Time It Was," 1 Sep. 2020 Disruptions to sleep and circadian rhythms are also linked to certain mental disorders, including bipolar disorder, mood swings, paranoia and anxiety. Sandee Lamotte, CNN, "Light pollution ruins teen sleep and may contribute to mental disorders, study says," 8 July 2020 Dr Foster suggests replacing high-sugar snacks with small protein-rich ones and lighting offices brightly enough to match employees’ circadian rhythms. The Economist, "Bedtime story Britons are dreaming more," 11 June 2020 Even a small light can disrupt a child’s circadian rhythms and serve as a bull’s-eye for the Gauntwings, who cannot hear or smell, and hunt their prey using only their hyperdeveloped sense of sight. Simon Rich, The New Yorker, "Everyday Parenting Tips," 18 May 2020

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'circadian.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of circadian

1959, in the meaning defined above

History and Etymology for circadian

Latin circa about + dies day + English -an entry 2 — more at deity

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The first known use of circadian was in 1959

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Last Updated

24 Oct 2020

Cite this Entry

“Circadian.” Dictionary, Merriam-Webster, Accessed 29 Oct. 2020.

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More Definitions for circadian


How to pronounce circadian (audio)

English Language Learners Definition of circadian

technical : relating to the regular changes in a person or thing that happen in 24-hour periods


cir·​ca·​di·​an | \ (ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē- How to pronounce circadian (audio) \

Medical Definition of circadian

: being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function) circadian periodicity circadian rhythms in behavior or physiological activity — compare infradian, ultradian

More from Merriam-Webster on circadian

Britannica English: Translation of circadian for Arabic Speakers

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