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 Anything to distract myself from the fact that another winter is about to set in and the circadian rhythm of the Bachelor Cinematic Universe has been disrupted. Ali Barthwell, Vulture, 21 Oct. 2021 Across all three experiments, the researchers found that a person’s circadian rhythm contributes to worsening asthma. Sara Harrison, Wired, 21 Sep. 2021 Working out increases levels of the mood-boosting brain chemical serotonin, which plays a role in regulating your circadian rhythms. Cassie Shortsleeve, Good Housekeeping, 27 Aug. 2021 Light from cities and ships also repels or attracts many species of fish, stresses coral reefs and disrupts the circadian rhythms of wood lice on sandy beaches. Scott Hershberger, Scientific American, 14 Aug. 2020 As cryptochrome became highly expressed in areas of cnidarian bodies that received a lot of light, variant copies of that gene evolved into photoreceptors that helped set the animals’ circadian rhythm. Quanta Magazine, 16 Aug. 2021 Our circadian rhythm—the body’s internal clock that regulates our sleeping and waking cycles—is influenced by light. Olivia F. Scott, Essence, 12 Aug. 2021 The Awaken Skylight harnesses light from above and helps with one's circadian rhythm. Bridget Degnan, Better Homes & Gardens, 11 Aug. 2021 Studies show that the disruption of the circadian rhythm cycle—the natural, internal process that regulates the sleep/wake cycle and repeats roughly every 24 hours—can be a risk factor for obesity, diabetes, cardiovascular disease and cancer. Bryan Robinson, Forbes, 11 Apr. 2021

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

25 Oct 2021

Cite this Entry

“Circadian.” Dictionary, Merriam-Webster, Accessed 28 Oct. 2021.

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More Definitions for circadian



English Language Learners Definition of circadian

: 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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