NREM (slow wave sleep)
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The first stage of sleeping, measurable as the replacement of alpha waves by theta waves when monitoring a subject using an EEG (electroencephalogram) machine. The subject being measured will present slow eye movements and may experience involuntary muscle contractions.
The second stage of sleeping, measurable by the appearance of sleep spindles ("sigma waves") and k-complexes. The subject will present little or no eye movement.
The third stage of sleeping, referred to as slow-wave sleep (SWS), measurable by the appearance of delta waves. Researchers following the Rechtschaffen and Kales (R&K) classification system divide this into two separate stages: in the first, delta waves represent 20-50% of activity; while in the second, they represent greater than 50%.