Circadian Rhythm
- “what is a circadian rhythm”
- “is the body clock exactly 24 hours”
- “circadian vs ultradian”
- “how does the body clock work”
:: 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 |
| Driven by | The SCN | Varies |
| Entrained by light | Yes | No |
| Best example | Sleep-wake | NREM-REM |
Which one you meant
Readers who land here meaning Circadian Rhythm rather than Ultradian are usually searching for phrases like “what is a circadian rhythm”, “is the body clock exactly 24 hours”, “circadian vs ultradian”.
Read either one in full