Circadian Rhythm
A circadian rhythm is the roughly 24-hour cycle of physical, mental and behavioural changes an organism runs on its own internal clock.
Full definition of Circadian Rhythm →:: COMPARE ::
CircadianvsUltradian
The short answer
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.
Side by side
| Aspect | 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 |
What each one means
Ultradian
The contrast above sets out how Ultradian differs from Circadian Rhythm on the points that actually matter.