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 rapidly changing light cycle can disrupt circadian rhythms, which can adversely affect awareness, concentration, and performance. —Jake Parks, Discover Magazine, 1 Mar. 2025

The natural circadian rhythms of humans and animals are disrupted by light pollution and have caused the decline of fireflies, the death of birds during migrations, and sea turtle confusion. —Cara Tabachnick, CBS News, 1 Mar. 2025

Your circadian rhythm—the body's internal clock—influences digestion, metabolism, and sleep quality. —Lauren Panoff, Mph, Rd, Verywell Health, 28 Feb. 2025

A lot of us have that afternoon dip and that’s part of the circadian rhythm, but that might fall at a different time for different people. —Allie Volpe, Vox, 25 Feb. 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