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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.


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

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