circadian

adjective

cir·​ca·​di·​an sər-ˈkā-dē-ən 

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

circadian rhythms in activity

Did you know?

In 1959, a scientist formed the word circadian from the Latin words circa ("about") and dies ("day"), and it caught on quickly. 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

Towey suggests trying something a little different on your next long-haul and downloading an app like Timeshifter, which prompts you to take small actions at specific times—like seeing bright light or avoiding caffeine—to adjust your circadian rhythm to a new time zone. —Jessica Sulima, Condé Nast Traveler, 17 Mar. 2026

John Hogenesch [a chronobiologist at Cincinnati Children’s Hospital] is very famous in this space for trying to implement circadian discoveries in clinical scenarios. —Lauren J. Young, Scientific American, 17 Mar. 2026

At that point, the parasite disrupts brain regions that regulate circadian rhythm, causing patients to become sleepy during the day and awake at night. —Fran Kritz, NPR, 16 Mar. 2026

Pass the legislation and stop messing with our circadian rhythms. —Voice Of The People, New York Daily News, 14 Mar. 2026

Word History

Etymology

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

First Known Use

1959, in the meaning defined above

Rhymes for circadian

• acadian
• akkadian
• arcadian
• palladian
• radian

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Medical Definition

circadian

adjective

cir·​ca·​di·​an (ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē- 

: 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