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

This clock determines your body’s daily cycle of sleep and alertness—the circadian rhythm. — Jd Shadel, Condé Nast Traveler, 22 Sep. 2026

Specialized cells in the eyes detect changes in light and communicate with the brain’s circadian system, helping coordinate processes such as alertness and sleep. — Ascend Agency, New York Daily News, 11 Sep. 2026

Future studies could also include measurements of the light spectrum and the body’s circadian rhythm. — Barbara Mantel, NBC news, 10 Sep. 2026

Don’t forget to open them in the morning to bathe in the daylight that helps set the body’s circadian rhythm. — Sandee Lamotte, CNN Money, 9 Sep. 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

• radian
• acadian
• akkadian
• arcadian
• palladian

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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