How many stages of non-REM sleep are there?

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BlackTom AI · Accepted Answer

The question asks about how many stages of non-REM sleep exist in typical sleep stage classifications.
Option 1: '4' — This reflects older conventions that listed four NREM stages (N1, N2, N3, and N4). Contemporary consensus often consolidates N3 and N4 into a single deep sleep stage, so counting four NREM stages is less accurate today.
Option 2: '3' — This matches the more current understanding where non-REM sleep comprises three distinct stages: N1, N2, and N3 (with N3 sometimes referred to as deep sleep). The progression from light sleep (N1) to intermediate (N2) and then to deep sleep (N3) reflects typical NREM architecture.
Option 3: '1' — Incorrect because non-REM sleep includes multiple identifiable stages rather than a single homogeneous state; treating it as one stage oversimplifies sleep structure.
Option 4: '2' — Also incorrect since non-REM sleep consists of more than two stages in standard classifications, covering at least light and deep sleep elements (N1, N2, and N3 in common formulations).
In summary, option 2 aligns with the standard three-stage model of non-REM sleep, while the other options either reflect outdated terminology or underestimate the known architecture of NREM sleep.

How many stages of non-REM sleep are there?

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