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

Jet lag occurs when your circadian rhythm falls out of sync due to flying across different time zones. —Angelina Villa-Clarke, Forbes.com, 28 Aug. 2025

This is because travel across time zones disrupts the body’s circadian rhythms, those natural patterns that regulate sleep, energy, digestion and other processes. —Austin Meek, New York Times, 22 Aug. 2025

Their routines—grounded in early morning prayer, structured meals, and limited screen use—align perfectly with best practices in circadian rhythm regulation. —Rabbi Bruce D. Forman, Sun Sentinel, 21 Aug. 2025

However, my data showed that consistent bedtimes and wake times helped align my circadian rhythm. —Allison Tsai, Verywell Health, 20 Aug. 2025

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