Figure 6.10: EEG waves of waking and sleep stages In general, as one goes from an alert to a relaxed state, and then to ever-deeper stages of sleep, the EEG waves become slower in frequency (fewer waves per second) and higher in amplitude (as shown by their greater vertical extent in the EEG record). The brief bursts of rapid waves called sleep spindles that appear in stage 2 are the most distinctive markers of the onset of sleep. Sleep stage 3 is not shown here; it is arbitrarily defined as the period when 10 to 50 percent of the EEG consists of delta waves. REM sleep, also not shown, is characterized by beta waves that look like those of the awake, attentive state.

(Adapted from Rechtschaffen & Kales, 1968.)