circadian
ser-KAY-dee-uhn
/sɜːrˈkeɪdiən/
Relating to biological processes that occur regularly on an approximately 24-hour cycle.
Circadian is almost exclusively used in biological contexts, particularly regarding sleep-wake cycles, hormone release, and body temperature.
The story behind the word. The term was coined in 1959 by the scientist Franz Halberg. He combined the Latin words 'circa' (meaning 'about' or 'around') and 'dies' (meaning 'day'). He wanted a precise word to describe biological cycles that were not exactly 24 hours, but 'about a day' in length when isolated from external cues.
Word relationships
- diurnal: 'Diurnal' means occurring during the daytime or active during the day, whereas 'circadian' refers specifically to the internal 24-hour cycle.
Commonly confused with
- daily: 'Daily' refers to events that happen every day in the external world, while 'circadian' refers to the internal biological cycle.
Word family
- circadianly (adverb): In a manner relating to a 24-hour biological cycle.
Collocations
- circadian rhythm
- circadian clock
- circadian biology
Example sentences
- "Traveling across multiple time zones disrupts your circadian rhythm, making it hard to sleep at night." Explaining the science of jet lag.
- "Blue light from smartphones can delay the circadian clock, suppressing melatonin production." From a scientific paper on light exposure.
Memory hook
Think of 'circa' (circle/around) and 'dian' (day/diary). A circle of a day. Circa-dian: Around a day.
When not to use
Do not use 'circadian' as a general synonym for 'daily' or 'everyday' (e.g., do not say 'my circadian chores'). It must refer to internal biological rhythms.
Fun facts
- The 2017 Nobel Prize in Physiology or Medicine was awarded to researchers who discovered the molecular mechanisms controlling the circadian rhythm.
Explore more words
nocturnal, crepuscular, periodic, daily, homeostasis, around the clock
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