Sleep Paralysis: Causes, Hallucinations and Prevention

Sleep paralysis is a brief state in which a person is awake and aware but temporarily unable to move or speak, usually just after waking up or while falling asleep. It is often accompanied by vivid, frightening hallucinations, such as a sense that someone is in the room, a crushing weight on the chest or the sight of a shadowy figure. Episodes usually last from a few seconds to a couple of minutes and end on their own. Sleep paralysis is generally considered physically harmless, but it can be terrifying, which is why it appears in folklore around the world as the "Old Hag", the "night-mare" and many other figures.

This page explains what happens in the brain during sleep paralysis, the kinds of hallucinations people report, who is most likely to experience it, how cultures have interpreted it, and what research suggests about prevention. It is educational information and not a substitute for a medical assessment.

What Sleep Paralysis Is

During REM (rapid eye movement) sleep, the stage most associated with vivid dreaming, the brain actively inhibits the body's voluntary muscles. This REM atonia prevents us from acting out our dreams. The muscles that control breathing and eye movement are spared, which is why breathing and rapid eye movements continue.

In sleep paralysis, the switch between sleep states is incomplete. A person regains conscious awareness while REM atonia is still in place, so the mind is awake but the body is not yet able to move. The opposite failure, in which atonia is missing during REM sleep and people physically act out dreams, is a different condition called REM sleep behavior disorder.

Researchers distinguish two timings:

• Hypnopompic sleep paralysis occurs while waking up and is the more commonly reported type.
• Hypnagogic sleep paralysis occurs while falling asleep, especially when a person enters REM sleep unusually quickly, as happens after sleep deprivation or in narcolepsy.

In the International Classification of Sleep Disorders, repeated episodes that are not explained by narcolepsy or another condition, and that cause distress, are classed as recurrent isolated sleep paralysis, one of the REM-related parasomnias.

How common is it?

In a systematic review of 35 studies, Brian Sharpless and Jacques Barber (2011) estimated that 7.6% of the general population have experienced sleep paralysis at least once, compared with about 28% of students and about 32% of psychiatric patients. Estimates vary widely between studies because of differences in how questions are asked, and many people experience only one or a few episodes in their lives.

The Three Types of Hallucinations

Hallucinations are common during sleep paralysis, although not every episode includes them. Canadian psychologist J. Allan Cheyne and colleagues (1999), drawing on surveys of thousands of people, grouped the experiences into three clusters.

Cheyne proposed that intruder and incubus experiences stem from the brain's threat-detection systems. Being unable to move is an alarming signal; the brain may respond by searching for a threat and generating a sense of one, which dream imagery then fills in. The chest pressure may relate partly to the shallow, regular breathing pattern of REM sleep combined with an inability to voluntarily take a deep breath. Vestibular-motor hallucinations may arise from conflicts between the brain's motor commands and the absence of feedback from the paralysed body.

The combination of a sensed intruder and a weight on the chest matches the "Old Hag" attack described in Newfoundland folklore and many similar traditions.

Why It Happens

Sleep is not a single on-off state. Different brain systems control wakefulness, REM sleep and non-REM sleep, and the transitions between them usually happen quickly and smoothly. Sleep paralysis seems to occur when those transitions are disrupted, leaving features of REM sleep (muscle atonia and dream imagery) mixed with features of wakefulness (awareness of the surroundings).

Several factors make this mixed state more likely:

• Sleep deprivation and REM pressure. After sleep loss, the brain tends to enter REM sooner and with more intensity, increasing the chance of awakenings out of REM.
• Fragmented or irregular sleep. Shift work, jet lag and irregular bedtimes disrupt the timing of sleep stages.
• Arousal and stress. Anxiety and stress are associated with more awakenings during the night.

The fear people feel is not simply a reaction to the hallucinations; it seems to be part of the experience itself, consistent with activation of emotional circuits such as the amygdala during REM sleep. Understanding this can make the fear easier to put in perspective.

Risk Factors and Triggers

A 2018 systematic review by Dan Denis, Christopher French and Alice Gregory identified a wide range of variables associated with sleep paralysis. The most consistent include:

• Poor sleep quality and sleep disruption, including insomnia symptoms and short sleep.
• Irregular schedules, such as shift work and jet lag.
• Stress and trauma exposure, with higher rates in people with PTSD and panic disorder.
• Anxiety symptoms, including general anxiety and anxiety sensitivity.
• Sleeping on the back. Cheyne's (2002) survey data found episodes were reported several times more often in the supine position than expected.
• Narcolepsy. Sleep paralysis is one of the four classic symptoms of narcolepsy, alongside excessive daytime sleepiness, cataplexy (sudden muscle weakness triggered by strong emotion) and hallucinations at sleep onset or waking.
• Family history. Twin studies suggest a modest genetic contribution.
• Substances and some medications that affect sleep architecture, as well as alcohol.

Adolescence and young adulthood are common times for first episodes, though sleep paralysis can occur at any age.

Cultural Interpretations

Sleep paralysis is one of the most cross-culturally consistent experiences in sleep research. People in very different societies describe similar sensations, including being unable to move, a malevolent presence and pressure on the chest, but interpret them through local beliefs.

The word "nightmare" itself originally referred to this experience: a "mare" was a spirit believed to sit on sleepers' chests. Henry Fuseli's 1781 painting The Nightmare, showing a demon crouched on a sleeping woman, is often interpreted as a depiction of sleep paralysis.

Modern interpretations have also drawn on sleep paralysis. Psychologists Richard McNally and Susan Clancy (2005) found that many people who reported being abducted by aliens described episodes consistent with sleep paralysis and hypnopompic hallucinations.

Culture can shape how distressing episodes are. Research by Devon Hinton and colleagues among Cambodian refugees, and studies in other groups, suggest that when sleep paralysis is interpreted as a supernatural attack or a sign of danger, it can be accompanied by more fear and lasting distress. Learning that it is a well-understood sleep phenomenon often reduces anxiety.

How to Stop an Episode

Episodes end on their own, usually within a minute or two. There is little controlled research on techniques to end an episode early, but people commonly report that the following help, mainly by reducing fear:

• Remind yourself what is happening. Thinking "this is sleep paralysis; it is harmless and will pass" can reduce panic.
• Focus on slow breathing. Breathing continues during episodes. Noticing this and slowing the breath can ease the sense of suffocation.
• Try small movements. Many people report that concentrating on moving a finger, a toe or the face helps them break out of the episode.
• Move your eyes. Eye muscles are not affected by REM atonia, and some people find looking around helps them wake fully.
• Avoid struggling. Fighting against the paralysis can intensify fear. Some people find that relaxing and letting the episode pass is less distressing.

Psychologist Baland Jalal has described a technique he calls meditation-relaxation therapy, which combines reappraising the episode as harmless, focusing attention inward and relaxing the muscles. It has been tested only in small preliminary studies so far.

Prevention

Because sleep paralysis is closely tied to sleep disruption, prevention focuses on sleep. These are general sleep and wellbeing practices; frequent or distressing episodes still deserve a professional assessment.

Regular, sufficient sleep

Most adults need around seven to nine hours of sleep. Keeping consistent bed and wake times, including on weekends, and avoiding cycles of sleep deprivation followed by oversleeping, reduces disruption of REM timing. See sleep hygiene for the research behind common recommendations.

Sleep position

Because episodes are reported more often when lying on the back, some people find that sleeping on their side reduces them. The evidence for this comes mostly from surveys rather than trials, but it is an easy change to try.

Reduce triggers

• Limit alcohol, especially in the hours before bed, since it disrupts REM sleep later in the night.
• Be mindful of caffeine, particularly in the afternoon and evening.
• For shift work and jet lag, plan light exposure and gradual schedule changes where possible.

Manage stress and anxiety

Stress management practices such as breathing exercises, progressive muscle relaxation and mindfulness may help some people sleep more calmly. When sleep paralysis occurs alongside panic attacks, PTSD or significant anxiety, treating those conditions is important in its own right.

Learn about it

Understanding what sleep paralysis is can reduce fear, which may in turn reduce sleep anxiety. Researchers such as Brian Sharpless have developed and piloted cognitive behavioral approaches specifically for recurrent isolated sleep paralysis that combine education, sleep changes and techniques for managing fear during episodes.

When to See a Doctor

Occasional sleep paralysis is common and usually needs no treatment beyond better sleep habits. Consider seeing a doctor or sleep specialist if:

• Episodes happen often or are becoming more frequent.
• You have excessive daytime sleepiness, which could suggest narcolepsy or another sleep disorder.
• You experience sudden muscle weakness triggered by laughter, surprise or other strong emotions.
• Fear of episodes is making you avoid sleep or causing significant distress.
• You have other sleep symptoms such as loud snoring, gasping or pauses in breathing, or severe insomnia.

A doctor can review sleep patterns, medicines and health conditions, and may arrange a sleep study. When sleep paralysis is frequent and distressing, treatment usually focuses on improving sleep and addressing anxiety; medication is one option a specialist may discuss in some cases, such as when narcolepsy is diagnosed.

Conclusion

Sleep paralysis is a common, usually harmless glitch at the boundary between REM sleep and wakefulness. For a short time, the mind is awake while the body remains in the paralysis of REM sleep, and dream imagery and threat systems can produce vivid, frightening hallucinations of intruders and chest pressure. Cultures throughout history have explained these experiences with hags, demons and spirits.

Today, sleep science offers a clearer picture: episodes are linked to sleep disruption, stress and sleeping position, and they end on their own. Regular sleep, stress management and understanding what is happening help many people. When episodes are frequent, distressing or accompanied by daytime sleepiness, a sleep assessment can identify conditions such as narcolepsy and guide appropriate care.