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
Your body follows a natural sleep-wake cycle (circadian rhythm) that responds to cues from light and darkness.
—
Vanessa Caceres, Verywell Health, 14 Jan. 2026
The morning is ideal for energy, routine, and maintaining a healthy circadian rhythm; the evening is beneficial for stress relief, sleep, and flexibility.
—Rikkilynn Shields Hannigan, Health, 14 Jan. 2026
Both Fischer and Lutron call out lighting systems and apps that change lighting based on pre-programmed modes and even automatically transition to align with circadian cycles.
—Kristina McGuirk, Better Homes & Gardens, 10 Jan. 2026
Researchers discovered that the light/dark cycle and the need to maintain stable conditions regulate sleep in jellyfish, whereas the circadian clock and internal timings primarily regulate sleep in sea anemones.
—Michelle Del Rey, USA Today, 9 Jan. 2026
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Vanessa Caceres, Verywell Health, 14 Jan. 2026
The morning is ideal for energy, routine, and maintaining a healthy circadian rhythm; the evening is beneficial for stress relief, sleep, and flexibility.
—Rikkilynn Shields Hannigan, Health, 14 Jan. 2026
Both Fischer and Lutron call out lighting systems and apps that change lighting based on pre-programmed modes and even automatically transition to align with circadian cycles.
—Kristina McGuirk, Better Homes & Gardens, 10 Jan. 2026
Researchers discovered that the light/dark cycle and the need to maintain stable conditions regulate sleep in jellyfish, whereas the circadian clock and internal timings primarily regulate sleep in sea anemones.
—Michelle Del Rey, USA Today, 9 Jan. 2026
See All Example Sentences for circadian
Cite this Entry
“Circadian.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/circadian. Accessed 17 Jan. 2026.
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circadian
adjective
cir·ca·di·an
(ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē-
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