circadian

What it means

1. not comparablebiologyOf, relating to, or showing rhythmic behaviour with a period of approximately 24 hours; especially of a biological process.

   The most obvious circadian rhythm is the daily cycle of sleep and activity.

   2005, Paul Martin, Counting Sheep: The Science and Pleasures of Sleep and Dreams, page 114

   4 more examples

   The circadian clock is involved in the regulation of the diurnal sleep-wake cycle, […].

   2000, Arnold Sameroff, et al: Handbook of Developmental Psychopathology, page 310

   To summarize, the circadian system, particularly the SCN, controls the circadian pattern of melatonin release in mammals.

   2002, Jill B. Becker, Behavioral Endocrinology, page 483

   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.

   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

   In addition, the truck driver’s schedule involved sleeping at irregular times, including times when the body’s circadian rhythms are geared toward being awake. He was also awake at times when his circadian rhythms were geared toward being asleep. The time of the crash, about 1:36 a.m., is when sleepiness tends to be greater and performance capabilities are lower.

   2024, NTSB, Highway Investigation Report, HIR-24-05

   Synonyms: hemeral, nyctohemeral, diurnal, quotidian, daily, diurnal, hemeral, nychthemeral

In a sentence

The circadian clock is involved in the regulation of the diurnal sleep-wake cycle, […].

2000, Arnold Sameroff, et al: Handbook of Developmental Psychopathology, page 310

Where it comes from

1. Proto-Indo-European*(s)ker-
2. Ancient Greekκίρκος (kírkos)
3. Latincircus
4. Latincircum
5. Latincircā
6. Proto-Indo-European*dyew-
7. Proto-Indo-European*dyḗws
8. Proto-Italic*djous
9. Latindiēs
10. Englishcircadian

From Latin circā (“around”) + diēs (“day”) + English -an. Compare circannual.