Sleep Mechanics: Sleep Disorders

Content provided by the Faculty of the Harvard Medical School

Sleep mechanics

For centuries, scientists scrutinized minute aspects of human activity, but showed little interest in the time that people spent in sleep. Sleep seemed inaccessible to medical probing and was perceived as an unvarying period of inactivity — a subject best suited to poets and dream interpreters who could conjure meaning out of the void. All that changed in the 1930s, when scientists learned to place sensitive electrodes on the scalp and record the signals produced by electrical activity in the brain. These brain waves can be seen on an electroencephalogram, or EEG (see Figure 1), which today is captured on a computer screen.

Figure 1: EEG brain wave patterns during sleep These brain waves, taken by electroencephalogram, are used by sleep experts to identify the stages of sleep. Close your eyes and your brain waves will look like the first band above, "relaxed wakefulness." Theta waves indicate Stage 1 sleep. Stage 2 sleep shows brief bursts of activity as sleep spindles and K-complex waves. Deep sleep is represented by large, slow delta waves (Stages 3 and 4).

After a few years of brain wave study, it became clear that sleep was a highly complex activity. Using electrodes to monitor sleepers' eye movements, muscle tone, and brain wave patterns, scientists now have identified several discrete stages of sleep. Researchers are continually learning more about the roles certain stages of sleep play in maintaining health, growth, and daytime functioning.

Scientists divide sleep into two major types: rapid eye movement (REM) sleep or dreaming sleep, and non-REM or quiet sleep. Surprisingly, they are as different from one another as sleeping is from waking.

Last updated:	January 23, 2007