The different types and stages of sleep can be best identified using polysomnography, which simultaneously measures several body functions such as brain wave activity (electroencephalogram or EEG), eye movement (electrooculogram or EOG), muscle activity (electromyogram or EMG), respiration, heart rhythm, etc. A simplified summary of these results can be combined into a graph called a hypnogram, which gives a useful visual cross-section of sleep patterns and sleep architecture (see the section on Sleep Measurement). Although the process of falling into a deeper and deeper sleep is a continuum, with no sharp delineating markers, these sleep stages provide a convenient, if somewhat simplistic, means of describing the depth and type of sleep as the sleep period progresses.
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.