Normal Sleep Stages

Abstract

Wakefulness (stage W) is characterized by the “alpha” rhythm, so named because it was the first electroencephalogram (EEG) rhythm discovered. Alpha frequency activity is 8–13 cycles/s and may be recorded anywhere in the brain. Alpha rhythm, however, has come to signify the sinusoidal resting rhythm of the occipital cortex. In addition to being in the above frequency range, it is usually of an amplitude around 30–50 mV and tends to occur in the posterior regions of the head, maximal over the occipital regions, and appears during relaxed wakefulness with eyes closed. In polysomnography interpretation, sleep onset is marked when the alpha rhythm attenuates and is absent from 50% of the epoch. The alpha rhythm, however, may attenuate and disappear when one is fully awake by just opening the eyes or thinking of a visual image. With sleep onset, the alpha frequency activity disappears not only from occipital channels but from all the electrodes (Figs. 1.1–1.6).