Sleep Disorders

A sleep disorder (somnipathy) is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning. A test commonly ordered for some sleep disorders is the polysomnogram.

Criteria for REM sleep include not only rapid eye movements, but also a rapid, low voltage EEG and, in mammals at least, low muscle tone. Most memorable dreaming occurs in this stage.

NREM accounts for 75–80% of total sleep time in normal human adults. In NREM sleep, there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep' or slow-wave sleep, SWS. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. There are often limb movements, and parasomnias such as sleepwalking occur in non-REM sleep. A cyclical alternating pattern may sometimes be observed during a stage.