Sleep paralysis is paralysis

Paralysis

Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

 associated with sleep that may occur in healthy persons or may be associated with narcolepsy

Narcolepsy

Narcolepsy is a chronic sleep disorder, or dyssomnia, characterized by excessive sleepiness and sleep attacks at inappropriate times, such as while at work. People with narcolepsy often experience disturbed nocturnal sleep and an abnormal daytime sleep pattern, which often is confused with insomnia...

, cataplexy

Cataplexy

Cataplexy is a sudden and transient episode of loss of muscle tone, often triggered by emotions. It is a rare disease , but affects roughly 70% of people who have narcolepsy...

, and hypnagogic hallucinations. The pathophysiology

Pathophysiology

Pathophysiology is the study of the changes of normal mechanical, physical, and biochemical functions, either caused by a disease, or resulting from an abnormal syndrome...

 of this condition is closely related to the normal hypotonia

Hypotonia

Hypotonia is a state of low muscle tone , often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength...

 that occurs during REM sleep. When considered to be a disease, isolated sleep paralysis is classified as MeSH

Medical Subject Headings

Medical Subject Headings is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching...

 D020188. Some evidence suggests that it can also, in some cases, be a symptom of migraine

Migraine

Migraine is a chronic neurological disorder characterized by moderate to severe headaches, and nausea...

.

Physiologically, sleep paralysis is closely related to REM atonia, the paralysis that occurs as a natural part of REM (rapid eye movement) sleep.