Sleep paralysis is a state, during waking up or falling asleep, in which one is conscious but is unable to move or speak. During an episode, one may hallucinate (hear, feel, or see things that are not there), which often results in fear. Episodes generally last no more than a couple of minutes. It can recur 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. Lucid dreaming does not affect the chances of sleep paralysis but some lucid dreamers use this as a method of having a lucid dream. 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% and 50% of people experience sleep paralysis at some point during their life. 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.