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

Late-night eating and shorter fasting periods may disrupt circadian rhythms and contribute to weight gain. —Mira Miller, Verywell Health, 19 May 2026

Sweetened coffee drinks fall into the same trap, especially when a sugar crash lands right on top of your circadian dip. —Allison Palmer, Kansas City Star, 19 May 2026

Per the American Heart Association’s scientific statement on circadian health, the body’s internal clock regulates cellular repair, hormone release and immune function on a precise schedule — and irregular timing throws that schedule off regardless of how many total hours are logged. —Allison Palmer, Charlotte Observer, 19 May 2026

For example, sleep architecture and circadian timing function as active healing levers. —Jesse Pines, Forbes.com, 18 May 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