Question 1

What is the difference between Circadian and Ultradian?

Accepted 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.

Question 2

How do I know if I mean Circadian Rhythm rather than Ultradian?

Accepted Answer

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”.

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

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Circadian Rhythm

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