From Kook Science

Non-Rapid Eye Movement (NREM) is series of stages that take place during the initial activity of sleep, so-called in contrast with Rapid Eye Movement sleep (REMS) that follows the NREM stages.

Stage N1

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.

Stage N2

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.

Stage N3

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%.