Sleep paralysis is a state, during waking up or falling asleep, in which a person is conscious but in a complete state of full-body paralysis. During an episode, the person may hallucinate (hear, feel, or see things that are not there), which often results in fear. Episodes generally last no more than a few minutes. It can recur multiple times or occur as a single episode.

The condition may occur in those who are otherwise healthy or those with narcolepsy, or it may run in families as a result of specific genetic changes. The condition can be triggered by sleep deprivation, psychological stress, or abnormal sleep cycles. The underlying mechanism is believed to involve a dysfunction in REM sleep. Diagnosis is based on a person's description. Other conditions that can present similarly include narcolepsy, atonic seizure, and hypokalemic periodic paralysis. Treatment options for sleep paralysis have been poorly studied. It is recommended that people be reassured that the condition is common and generally not serious. Other efforts that may be tried include sleep hygiene, cognitive behavioral therapy, and antidepressants.

Between 8% to 50% of people experience sleep paralysis at some point during their lifetime. About 5% of people have regular episodes. Males and females are affected equally. Sleep paralysis has been described throughout history. It is believed to have played a role in the creation of stories about alien abduction and other paranormal events.

The main symptom of sleep paralysis is being unable to move or speak during waking.

Imagined sounds such as humming, hissing, static, zapping and buzzing noises are reported during sleep paralysis. Other sounds such as voices, whispers and roars are also experienced. It has also been known that one may feel pressure on their chest and intense pain in their head during an episode. These symptoms are usually accompanied by intense emotions such as fear and panic. People also have feelings of drowning or sinking, being dragged out of bed or of flying, numbness, and feelings of electric tingles or vibrations running through their body.

Sleep paralysis may include hallucinations, such as an intruding presence or dark figure in the room. These are commonly known as sleep paralysis demons. It may also include the inability to breathe or the individual feeling a sense of terror, accompanied by a feeling of pressure on one's chest and difficulty breathing.

The pathophysiology of sleep paralysis has not been concretely identified, although there are several theories about its cause. The first of these stems from the understanding that sleep paralysis is a parasomnia resulting from dysfunctional overlap of the REM and waking stages of sleep. Polysomnographic studies have found that individuals who experience sleep paralysis have shorter REM sleep latencies than normal along with shortened NREM and REM sleep cycles, and fragmentation of REM sleep. This study supports the observation that disturbance of regular sleeping patterns can precipitate an episode of sleep paralysis, because fragmentation of REM sleep commonly occurs when sleep patterns are disrupted and has now been seen in combination with sleep paralysis.

Another major theory is that the neural functions that regulate sleep are out of balance, causing different sleep states to overlap. In this case, cholinergic sleep "on" neural populations are hyperactivated and the serotonergic sleep "off" neural populations are under-activated. As a result, the cells capable of sending the signals, that would allow for complete arousal from the sleep state, the serotonergic neural populations, have difficulty in overcoming the signals sent by the cells that keep the brain in the sleep state. During normal REM sleep, the threshold for a stimulus to cause arousal is greatly elevated. Under normal conditions, medial and vestibular nuclei, cortical, thalamic, and cerebellar centers coordinate things such as head and eye movement, and orientation in space.