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.
Example Sentences
Recent Examples on the Web Blue light exposure has been previously linked to disruptions in the circadian rhythm and people’s sleep cycles, according to Harvard Medical School. 
Alexa Mikhail, Fortune, 16 Sep. 2022 Blue light glasses and brightness reduction technology are just two options for working to reset your circadian rhythm and other DIY sleep enhancement tips. Zee Krstic, Good Housekeeping, 9 Sep. 2022 The most critical circadian rhythm, and the book’s primary focus, is the sleep/wake cycle, which too often falls out of alignment in our high-wattage, 24/7 world. A. Roger Ekirch, WSJ, 28 Aug. 2022 Plus, according to the National Sleep Foundation, exposure to bright lights and screens before bed can throw off your circadian rhythm and compromise your sleep quality. Abigail Bailey, Good Housekeeping, 5 Aug. 2022 Internal biological clocks, also known as circadian rhythms, are universally shared by living organisms, according to the University of Illinois Urbana-Champaign’s Agricultural, Consumer & Environmental Sciences News. Jacob Livesay, USA TODAY, 26 July 2022 Bright lights in cities can draw some species of birds away from their migratory routes, and exposure to blue light at night can disrupt humans' circadian rhythms, hurting sleep quantity and quality. Madeline Heim, Journal Sentinel, 28 June 2022 The studies also show eye disorders and poor health usually come hand in hand — as eye problems seem to correlate to the body’s circadian rhythm, the natural process that regulates the sleep-wake cycle. Ebony Williams, ajc, 17 June 2022 The cells in our eyes that set the circadian rhythms respond best to the sun, so morning sunlight is particularly effective, starting a timer in the brain that sets the stage for the sleep that will come 15 or so hours later. Julie Wright, WSJ, 26 Mar. 2022 See More
Alexa Mikhail, Fortune, 16 Sep. 2022 Blue light glasses and brightness reduction technology are just two options for working to reset your circadian rhythm and other DIY sleep enhancement tips. Zee Krstic, Good Housekeeping, 9 Sep. 2022 The most critical circadian rhythm, and the book’s primary focus, is the sleep/wake cycle, which too often falls out of alignment in our high-wattage, 24/7 world. A. Roger Ekirch, WSJ, 28 Aug. 2022 Plus, according to the National Sleep Foundation, exposure to bright lights and screens before bed can throw off your circadian rhythm and compromise your sleep quality. Abigail Bailey, Good Housekeeping, 5 Aug. 2022 Internal biological clocks, also known as circadian rhythms, are universally shared by living organisms, according to the University of Illinois Urbana-Champaign’s Agricultural, Consumer & Environmental Sciences News. Jacob Livesay, USA TODAY, 26 July 2022 Bright lights in cities can draw some species of birds away from their migratory routes, and exposure to blue light at night can disrupt humans' circadian rhythms, hurting sleep quantity and quality. Madeline Heim, Journal Sentinel, 28 June 2022 The studies also show eye disorders and poor health usually come hand in hand — as eye problems seem to correlate to the body’s circadian rhythm, the natural process that regulates the sleep-wake cycle. Ebony Williams, ajc, 17 June 2022 The cells in our eyes that set the circadian rhythms respond best to the sun, so morning sunlight is particularly effective, starting a timer in the brain that sets the stage for the sleep that will come 15 or so hours later. Julie Wright, WSJ, 26 Mar. 2022 See More Word History
Etymology
Latin circa about + dies day + English -an entry 2 — more at deity
First Known Use
1959, in the meaning defined above
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