During the transition from wakefulness to sleep, polysomnography shows low-amplitude mixed-frequency EEG with prominent high-voltage sharp waves over the central regions occurring singly or in runs. These transients define which sleep stage?
- A Stage N2
- B Stage N1 ✓
- C Stage N3
- D REM sleep
Correct answer: B. Stage N1
Explanation
Vertex sharp waves are high-amplitude negative deflections maximal at the central electrodes and are a defining feature of stage N1, together with low-amplitude mixed-frequency activity replacing the posterior alpha rhythm. Stage N2 requires spindles or K-complexes, stage N3 requires more than 20 percent delta activity, and REM requires a desynchronised EEG with rapid eye movements and atonia.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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