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

Take naps whenever necessary to get your circadian rhythms back to normal. —Nafeesah Allen, Better Homes & Gardens, 1 Aug. 2025

While Labor Day marks the unofficial end of summer for many, your body's circadian rhythm may not recognize the shift to shorter, colder days until the end of daylight saving time. —Alexis Simmerman, Austin American Statesman, 31 July 2025

Advertisement Read More: The Race to Explain Why More Young Adults Are Getting Cancer One factor is the body’s circadian rhythms. —Time, 31 July 2025

By contrast, doing some movement in the back half of the day may delay your circadian rhythm a bit, which could benefit early birds who are looking to stay awake later into the night. —Erica Sloan, SELF, 30 July 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

Podcast

Get Word of the Day delivered to your inbox!

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