cir·​ca·​di·​an | \sər-ˈkā-dē-ən \

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

The researchers believe that a shifted sleep schedule affects circadian rhythm and hormone levels throughout the day, and that throwing them out of whack could contribute to both physical and emotional health issues. Amanda Macmillan,, "Sleeping In on the Weekend Might Actually Be Good For You," 24 May 2018 There’s also some evidence that switching sleeping schedules twice a year screws up our circadian rhythms leading to adverse health effects. Jason Daley, Smithsonian, "The EU Mulls Ditching Daylight Saving Time," 9 July 2018 Critics argue that switching up the clocks drives up electricity demand and disrupts human’s delicate circadian rhythms, leading to long-term health problems. Shoshana Wodinsky, The Verge, "The EU is polling citizens if daylight saving is really necessary," 5 July 2018 Mad increases in daylight cue chemical changes in the brain that enable animals to ditch their circadian rhythms, Kielland said. Ned Rozell, Anchorage Daily News, "During long summer days in Alaska, Boreal owls perform by daylight," 30 June 2018 The rodents are part of a study to determine how living on a sterile ship in the dark vacuum of space alters circadian rhythms—and potentially disrupts a healthy microbiome. Megan Molteni, WIRED, "NASA's Astro-Mice Will Test What Space Does to Your Gut," 28 June 2018 Researchers said chemical exposure and regular disruption of employees' circadian rhythms could be to blame. James Hohmann, Washington Post, "The Daily 202: Crowley going down spotlights the looming Democratic identity crisis," 27 June 2018 Previous studies have found a link between a person’s unique circadian rhythm, or chronotype, and depression, but none were able to tell whether sleep habits were a cause or an effect of the disease. Nicholas Bakalar, New York Times, "Night Owls May Have Higher Depression Risk," 20 June 2018 The findings do show a physiological disconnect — many of the genes associated with circadian cycles kept their rhythms, but seemed to fire at lower strength. Megha Satyanarayana, STAT, "Stubborn genes: New research looks at how our bodies respond (or don’t) to night shift work," 7 May 2018

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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Last Updated

17 Sep 2018

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The first known use of circadian was in 1959

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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- \

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

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Britannica English: Translation of circadian for Arabic Speakers

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What made you want to look up circadian? Please tell us where you read or heard it (including the quote, if possible).


to clear from alleged fault or guilt

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