A circadian rhythm is the roughly 24-hour cycle of physical, mental and behavioural changes an organism runs on its own internal clock.
Coined in 1959 by the German-born biologist Franz Halberg, from Latin circa ('about') + dies ('day').
// Where it sits
The imprecision is in the word on purpose, and it's why sunlight matters.
What it really means
The most important word in circadian is about.
Circā diēs — around a day. The imprecision is deliberate and it encodes a real fact: your internal clock does not run on exactly 24 hours. In humans it runs slightly longer — night-active mammals run slightly short — which means that left entirely to itself a human clock drifts steadily later, day after day, until you were sleeping through the afternoon.
What stops that is light. Daily light exposure entrains the clock — nudges it back into alignment with the actual rotation of the planet. The mechanism isn't decorative; it's a correction applied every single morning to a timer that would otherwise wander off.
The 2017 Nobel is often described as being awarded for discovering circadian rhythms, which is wrong by nearly three centuries — de Mairan had a mimosa plant folding its leaves on schedule in constant darkness in 1729. The prize was for the molecular mechanism: a feedback loop in which the PER protein accumulates through the night, then degrades through the day, with the period and timeless genes suppressing their own production. A clock built out of a protein that inhibits its own manufacture.
And one distinction worth keeping: your biological clock is the machinery, while circadian rhythms are what it produces. The suprachiasmatic nucleus is the clock. Feeling alert at 10am is the rhythm.
Where it comes from
The 'about' is load-bearing and deliberate. The word encodes the fact that the internal period is NOT exactly 24 hours — day-active species run slightly long, night-active ones slightly short — which is why the clock must be reset daily by light.
Myths & misconceptions
Your body clock runs on exactly 24 hours.
It runs on approximately 24 hours — slightly more than 24 in humans and other day-active species, and slightly less in night-active ones. The word circadian literally means 'about a day' for exactly this reason. Daily light exposure entrains the clock and corrects the drift; without it, sleep timing slides later.
The 2017 Nobel was awarded for discovering that circadian rhythms exist.
Rhythms were known long before. The prize was for the molecular mechanism — the PER protein accumulating overnight and degrading during the day, with the period and timeless genes forming a self-repressing feedback loop.
Compare & contrast
Circadian vs ultradian rhythm
Defined purely by period length. Circadian is about a day; ultradian is less than a day, like the roughly 90-minute sleep cycle; infradian is more than a day. Same taxonomy, three brackets — the classification is by period alone, though the mechanisms are not interchangeable: circadian rhythms run on a light-entrained master clock, and most ultradian ones do not.
| // | Circadian | Ultradian |
|---|---|---|
| Period | ~24 hours | Under 24 hours |
| Reset by | Light | Not clock-driven |
| Master clock | The SCN | Various |
| Example | Sleep-wake | NREM-REM cycle |
How it connects
- Ultradian Rhythmthe faster sibling, and the one with a confusing prefix.
- Phenologyboth study biological timing, one within a day and one across a year.
Tell it apart
Questions people ask
- Who won the 2017 Nobel for circadian rhythms?
- Jeffrey C. Hall, Michael Rosbash and Michael W. Young, for discovering the molecular mechanisms that control them — specifically the feedback loop in which the PER protein accumulates overnight and degrades during the day.
- Where is the body clock?
- In vertebrates the master clock is the suprachiasmatic nucleus, a group of nerve cells in the brain. Individual tissues run their own clocks too, which the SCN keeps synchronised.
- Is a body clock the same as a circadian rhythm?
- No. The biological clock is the regulating system; circadian rhythms are the physical, mental and behavioural changes it produces. One is the mechanism, the other the output.