A 45-year-old patient undergoing polysomnography shows high-amplitude, synchronous, low-frequency EEG waves (0.5–2 Hz) with no eye movements and no muscle atonia. In which sleep stage is this patient MOST likely?
- A Stage N1 (NREM 1)
- B Stage N3 (slow-wave sleep / deep NREM) ✓
- C Stage N2 (NREM 2)
- D REM sleep
Correct answer: B. Stage N3 (slow-wave sleep / deep NREM)
Explanation
Stage N3 (slow-wave sleep, SWS) is characterized by delta waves — high amplitude (>75 µV), slow frequency (0.5–2 Hz), synchronous waves constituting >20% of the epoch. There are no rapid eye movements and muscle tone is present (unlike REM). N1 shows theta waves (4–7 Hz); N2 shows sleep spindles and K-complexes; REM shows low-amplitude, mixed-frequency EEG with muscle atonia and REMs.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.