Why is the cycle of sleep stages classed as an ultradian rhythm?

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Question

Why is the cycle of sleep stages classed as an ultradian rhythm?

Accepted Answer

One complete pass through the sleep stages takes around 90 minutes, so the cycle repeats several times within a single night rather than over 24 hours — Award 1 mark. Later cycles contain proportionally more REM sleep.

Answer

It is controlled by melatonin alone

Answer

One complete pass through the sleep stages takes around 90 minutes, so the cycle repeats several times within a single night rather than over 24 hours

Answer

It lasts longer than a day

Answer

It repeats only once each night

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