FoundationalConceptsNeuroscience

In humans and some other mammals, NREM sleep can be divided
into several stages that differ by the kind of EEG activity, labeled
stages 1, 2, 3, and 4. Stages 3 and 4 are recognized by EEG patterns
that contain substantial low-frequency oscillation, less than 4 hertz
(delta waves), and thus these stages are sometimes referred to as slow-
wave sleep. During REM sleep, EEG activity is dominated by higher
frequencies and looks more like the EEG of an awake person than it
does the EEG of a person in NREM sleep.
EEG measurements in sleeping people demonstrate that there is
a fairly regular progression through these various stages over the
course of a night’s sleep (Fig. 20.1). After sleep onset, we go through
a progression of the four NREM stages. After this, there is movement
back up the stages, and you might get the idea that the sleeper is going
to awaken, but instead the sleeper enters a period of REM sleep. In
REM sleep, the electrooculograph shows movement of the eye mus-
cles. If you look carefully at the closed eyelids of a person in REM sleep,
you can see the twitching of the eyeball beneath the eyelid. The eye-
ball will typically move back and forth once every second or two. After
a few minutes the eyes stop twitching, the EEG slows down again,
and the sleeper passes back through the various NREM stages. Similar
patterns are repeated throughout the night, with the periods of REM
sleep becoming longer and more frequent as the night progresses.