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From the Latin circā (about) and diēs (a day) with the English suffix -an. Compare circannual.


circadian (not comparable)

  1. (biology) of, relating to, or showing rhythmic behaviour with a period of 24 hours; especially of a biological process
    • 2000, Arnold Sameroff, et al., Handbook of Developmental Psychopathology, page 310
      The circadian clock is involved in the regulation of the diurnal sleep-wake cycle, […].
    • 2002, Jill B. Becker, Behavioral Endocrinology, page 483:
      To summarize, the circadian system, particularly the SCN, controls the circadian pattern of melatonin release in mammals.
    • 2005, Paul Martin, Counting Sheep: The Science and Pleasures of Sleep and Dreams, page 114:
      The most obvious circadian rhythm is the daily cycle of sleep and activity.
    • 2014 April 5, “Quite interesting: A quietly intriguing column from the brains behind QI, the BBC quiz show. This week; QI orchids you not”, in The Daily Telegraph (Weekend), page W22:
      Circadian clocks developed early in evolution in single-celled organisms, before the animal and plant kingdom split from one another. The original clocks probably functioned to protect the cells from damage induced by high UV radiation.

Derived termsEdit

Related termsEdit


  • diurnal (in its sense of "happening on a 24-hour cycle")


See alsoEdit