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 answer lies in your circadian rhythm—your body’s internal clock that regulates energy levels, alertness, and cognitive performance throughout the day. —Luciana Paulise, Forbes.com, 24 May 2025

Pros and cons of taking melatonin or another sleep aid while flying From a scientific standpoint, melatonin is most useful not as a sleep aid per se, but as a tool for circadian realignment. —Bailey Berg, AFAR Media, 23 May 2025

Health problems, prescription medication side effects and changes in circadian rhythms are among many reasons people struggle with getting good rest. —The Washington Post, San Diego Union-Tribune, 20 May 2025

His exhaustive training and reminders to eat have helped regulate his circadian rhythm, and the impact has been noticeable. —Christopher Cruz, Rolling Stone, 20 May 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