There are two main broad types of sleep, each with its own distinct physiological, neurological and psychological features: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM or NREM) sleep, the latter of which can in turn be divided into three or four separate stages. Non-REM sleep is sometimes referred to as “quiet sleep” and REM as “active sleep”, although these are not scientific terms.
Electroencephalogram (EEG) traces of brain wave activity reveal that, in general terms, non-REM sleep is characterized by very slow but relatively high amplitude or high voltage oscillations (with the frequency gradually slowing and the amplitude increasing as sleep deepens), while REM sleep shows a much faster and lower amplitude trace, much more similar to normal waking activity (see diagram above). Also, brain waves during non-REM sleep tend to be highly synchronized, and those during REM sleep much more unsynchronized. Electrooculogram (EOG) traces of eye movement indicate rapid eye movements during REM sleep, and little or no eye movement during non-REM sleep. Electromyogram (EMG) traces of skeletal muscle activity show that, while the body is effectively completely paralyzed during REM sleep, the body does make some limited movements during non-REM sleep, including a major change in body position about once every twenty or thirty minutes on average. Based on these characteristics, early sleep researcher William C. Dement has described non-REM sleep as an idling brain in a moving body, and REM sleep as an active hallucinating brain in a paralyzed body.