Relaxed wakefulness with the eyes closed is accompanied in the electroencephalogram (EEG) by posterior alpha waves of 9 to 11 Hz (cycles per second) and intermixed low-voltage fast activity of mixed frequency. Except for the facial muscles, the electromyogram (EMG) is silent when the patient is sitting or lying quietly. With drowsiness, as the first stage of sleep sets in, the eyelids begin to droop, the eyes may rove slowly from side to side, and the pupils become smaller. As the early stage of sleep evolves, the muscles relax and the EEG pattern changes to one of progressively lower voltage and mixed frequency with a loss of alpha waves; this is associated with slow, rolling eye movements and is stage 1 sleep. With progression to stage 2 sleep, there is the appearance of 0.5to 2-s bursts of biparietal 12to 14-Hz waves (sleep spindles) and intermittent high-amplitude, central-parietal sharp slow-wave complexes (vertex waves) (Fig. 18-1). Stage 3 represents slow-wave sleep with predominant theta rhythms and stage 4, deep slow wave sleep with a preponderance of delta frequency activity. Vertex waves and sleep spindles are no longer evident.
If the eyelids are raised gently during sleep, the globes are usually seen to be exotropic and the pupils are even smaller than before, but with retained responses to light. An additional stage of the sleep cycle, which follows the others intermittently throughout the night, is associated with further reduction in muscle tone except in the extraocular muscles and with bursts of rapid eye movement; thus the term rapid eye movement (REM) sleep designates this stage. The EEG becomes desynchronized, that is, it has a low-voltage, high-frequency discharge pattern. The first three stages of sleep are called nonrapid eye movement (NREM) sleep or synchronized sleep; the last stage, in addition to REM sleep, is variously designated as fast-wave, nonsynchronized, or desynchronized sleep. Figure 18-2 illustrates these features.
The American Academy of Sleep Medicine now recommends the following nomenclature: stage W (wakefulness), stage N1 (non-REM sleep, or NREM 1, formerly stage 1), stage N2 (NREM 2, formerly stage 2), stage N3 (NREM 3, combining former stages 3 and 4—or slow-wave sleep), and stage R (REM sleep) (Fig. 18-3). The essential difference between this new nomenclature and the one formerly used by neurologists is that stage N3 now represents slow-wave sleep, replacing stage 3 and stage 4 sleep, composed of an increasing proportion of high-amplitude delta waves (0.75 μV, 0.5 to 2 Hz) in the EEG (Table 18-1).
In the first portion of a typical night’s sleep, the normal young and middle-aged adult passes successively through stages N1, N2, N3, and R (REM) sleep. After about 70 to 100 min, a large proportion of which consists of stage N3 sleep, the first REM period occurs, usually heralded briefly by a transient increase in body movements and a shift in the EEG pattern from that of stage N3 to stage N2. This NREM–REM cycle is repeated at about the same interval four to six times during the night, depending on the total duration of sleep. The first REM period may be brief; the later cycles have less stage N3 sleep or none at all. In the latter portion of a night’s sleep, the cycles consist essentially of two alternating stages—REM sleep and stage N2 (spindle–K-complex) sleep. The relation of dreaming to these sleep stages is described further on.