TABLE 9-7 - Sleep Stages

Nonrapid eye movement (NREM)

Stage 0	Awake.
Stage 1	Very light* sleep, transition from wakefulness to sleep. Drowsy.
Stage 2	Medium depth of sleep, occupies about half the night in adults. Serves as a transition stage between rapid eye movement (REM) and delta sleep. EEG demonstrates sleep spindles and k-complexes.
Delta	Slow wave sleep, composed of stages 3 and 4, occupies 10-30% oftotal sleep.
Stage 3	Consists of a moderate amount of delta wave activity; deeper sleep than stage 2.
Stage 4	Increased delta wave activity over stage 3. Very deep stage of sleep.
REM	Dream sleep. EEG is active, mimicking that of the waking stage. Depth of sleep is greater than stage 2 but probably less than delta.

* Depth of sleep as used here is not a precise term but generally refers to ease of arousability (i.e., how hard would it be to awaken an individ- ual from a particular stage). However, the ease of arousability is due in part to the type of stimulus used (e.g, noise versus touch).