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

The circadian system refers to the body’s suite of clocks—every cell with DNA has a clock, and each of these clocks feeds back into one another. —Claire Cameron, Scientific American, 8 Mar. 2026

There is also a strong focus on the circadian rhythm, your body’s normal sleep/wake cycles that responds to light exposure. —Emilee Coblentz, Outside, 8 Mar. 2026

There are ways to ease the adjustment, including getting more sunshine to help reset your circadian rhythm for healthful sleep. —Lauran Neergaard, Chicago Tribune, 7 Mar. 2026

Daily exposure to sunlight helps regulate your circadian rhythm, which influences sleep quality, insulin sensitivity, appetite, and glucose (sugar) control. —Brittany Poulson, Verywell Health, 7 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