SLEEPING

Sleeping

Sleeping

What is sleep?

Sleep is defined in the laboratory by the use of three standard psychophysical measures: electroencephalography (EEG), which records brain wave activity; electrooculography (EOG), which records eye movements; and electromyography (EMG), which measures chin muscle activity, giving an indication of muscle tonus (Empson, 2002). Information from these instruments indicates sleep and sleep stages. During periods of wakefulness, the EEG pattern shows two basic types of activity. Beta waves occur when the individual is mentally alert and active. This is characterized by low-amplitude, irregular, high-frequency waves (13+ Hz). Alpha waves occur when an individual is resting quietly and is particularly evident when the eyes are closed. Alpha waves are high amplitude, more regular, and lower frequency (8-12 Hz).

There are two types of sleep: non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Within NREM sleep there are four stages usually recognized in humans. (Ellman & Antrobus, 1991)

Stage 1 is the transitional phase between full wakefulness and sleep. EEG activity is theta waves (3-7 Hz). In normal sleepers, stage 1 lasts from 30 seconds to about 7 minutes. During this period, reactivity to outside stimuli is diminished. Mental processes change during stage 1 as thoughts begin to drift, thinking is no longer reality oriented, and short dreams often develop. This is usually accompanied by slow, rolling eye movements. Nevertheless, many people subjectively feel that they are awake during stage 1.

Stage 2 continues to show theta waves and is marked by the appearance of EEG sleep spindles (bursts of 12-14 Hz activity lasting 0.5 second to 2 seconds) and k complexes (well delineated, slow negative EEG deflections that are followed by a positive component). This is the first bona fide sleep stage, and any thoughts during this stage are short, mundane, and fragmented. Slow-rolling eye movements cease when stage 2 begins.

Stage 3 ...