Ultradian Rhythm
An ultradian rhythm is a biological cycle that repeats more than once a day — any rhythm with a period shorter than 24 hours.
Full definition of Ultradian Rhythm →:: COMPARE ::
UltradianvsCircadian
The short answer
Purely a matter of period. Circadian is about 24 hours and driven by a master clock entrained by light; ultradian is anything faster, and the best-established example — the NREM-REM cycle — is sleep-dependent rather than clock-driven.
Side by side
| Aspect | Ultradian | Circadian |
|---|---|---|
| Period | Under 24h | ~24h |
| Driven by | Varies | The SCN |
| Entrained by light | No | Yes |
| Best example | NREM-REM | Sleep-wake |
What each one means
Circadian
The contrast above sets out how Circadian differs from Ultradian Rhythm on the points that actually matter.