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

Because our colon operates on a circadian rhythm, some of the greatest contractions of the day occur in first hour or so after waking. —Kristen Rogers, CNN Money, 7 Apr. 2026

Similarly, a 2026 clinical trial found that stopping eating at least three hours before bedtime helped better align the body’s circadian rhythms with sleep, improving nighttime blood pressure, heart rate and blood sugar regulation. —The Washington Post, San Diego Union-Tribune, 7 Apr. 2026

Get Morning Sunlight Many people spend much of the day inside at work or home exposed to artificial lights, and this can mess with the body’s circadian rhythm. —Kristen Gasnick, Verywell Health, 3 Apr. 2026

Due to your circadian rhythm, your blood pressure naturally dips by 10–20% in the evening and during sleep. —Jenessa Connor, Health, 2 Apr. 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