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

Our bodies follow a specific 24-hour cycle (your circadian rhythm), and our health and hormones need to be in sync with that. —Sohaib Imtiaz, Verywell Health, 24 Nov. 2025

Odors act as circadian time cues that help synchronize the body’s biological clock and signal the time of year. —Paul Nicolaus, Outside, 22 Nov. 2025

Research shows that sleeping in nature can reset your circadian rhythm, even for those who’ve adopted poor nighttime habits. —Stephanie Vermillion, Vogue, 12 Nov. 2025

Brigham and Women’s Hospital has been at the forefront of sleep and circadian rhythm research for nearly a century. —Alice Gibbs, MSNBC Newsweek, 10 Nov. 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