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

But for many others, sleep problems run much deeper as millions of people live with sleep disorders such as insomnia, restless legs syndrome, circadian rhythm disorders or sleep apnea. — Daryl Austin, USA Today, 8 July 2026

Jamie Zeitzer, a professor of sleep medicine at Stanford University who studies circadian rhythms, said that sleep timing and regularity are in many ways as important for your health and well-being as the duration of your sleep. — The Washington Post, San Diego Union-Tribune, 7 July 2026

Examples include electrical stimulation to calm the nervous system, insulin delivered in response to rising glucose and light therapy to shift circadian rhythms. — Samantha Agate, Miami Herald, 6 July 2026

When Coffee Has Its Biggest Impact The best time to indulge in that creamy latte or espresso shot often depends on your natural energy levels, your circadian rhythm, and how sensitive your body is to caffeine. — Allison Forsyth, Health, 1 July 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