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 matters because humans have a direct neural pathway from the eyes to the brain region that regulates the circadian rhythm, our internal biological clock governing wakefulness and sleep. — Sharon Brandwein, USA Today, 19 June 2026

Jennifer Martin, a professor at Florida International University's College of Medicine, explains that chronotypes are driven by the circadian rhythm—the body's internal clock that regulates sleep, alertness, and other essential daily functions. — Olivia McIntosh, Martha Stewart, 19 June 2026

Morning or daytime sessions support your circadian rhythm because red light is mitochondrially stimulating and late-night use can disrupt sleep. — Allison Palmer, Miami Herald, 18 June 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, 18 June 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