circadian

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")

• circadian rhythm
• circadian clock

• circalunar
• circasemidian

• infradian
• ultradian

• cardician