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

Both Fischer and Lutron call out lighting systems and apps that change lighting based on pre-programmed modes and even automatically transition to align with circadian cycles. —Kristina McGuirk, Better Homes & Gardens, 10 Jan. 2026

Researchers discovered that the light/dark cycle and the need to maintain stable conditions regulate sleep in jellyfish, whereas the circadian clock and internal timings primarily regulate sleep in sea anemones. —Michelle Del Rey, USA Today, 9 Jan. 2026

This hormone helps guide the body’s circadian rhythm, so boosting your cortisol levels before bed can be a major sleep disruptor. —Hunter Boyce, AJC.com, 8 Jan. 2026

The participants also showed a shift in their circadian rhythms (sleep/wake cycles) when they were placed on the time-restricted eating schedules, but the associated health impacts are not known. —Melissa Rudy, FOXNews.com, 7 Jan. 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