Sleep Paralysis: The Science Behind the Night Visitor

The experience usually begins on waking, or just as sleep arrives. The person is fully aware, with eyes open and the bedroom looking exactly as it should, but cannot move a limb or make a sound. Then, often, comes the sense that someone else is in the room. People describe footsteps, whispering or breathing, a shadowy figure in the doorway or standing by the bed, and sometimes a weight on the chest that makes it hard to draw a full breath, as if someone were sitting there. Fear is common, sometimes overwhelming. The episode usually lasts from a few seconds to a few minutes and ends on its own.

Many people who have had sleep paralysis never tell anyone, worried that they will not be believed or that something is wrong with them. Others describe it as the most frightening thing that has ever happened to them, and some interpret it as a visit from a spirit or demon, as people have for centuries.

Two things about it stand out to researchers. The first is how real it feels: this is not a vague dream remembered later, but an experience that seems to happen in the actual room, while the person knows they are awake. The second is how similar the descriptions are across cultures and centuries, from early modern Europe to Japan to Newfoundland. That consistency is one of the strongest clues to what is going on.

During REM sleep, the stage associated with the most vivid dreams, the brain switches off most of the body's voluntary muscles so that we do not act out our dreams. The eyes and the muscles that keep us breathing go on working. Sleep paralysis happens when awareness returns, or arrives early, while that REM paralysis and some of the dream imagery are still in place. The result is a mixed state, partly waking and partly dreaming. Hallucinations as sleep begins are called hypnagogic, and those on waking hypnopompic.

The psychologist J. Allan Cheyne and colleagues, analyzing large surveys of people who have had sleep paralysis, sorted the hallucinations into three broad groups. The intruder group includes the sensed presence, sounds and shadowy figures. The incubus group covers pressure on the chest, difficulty breathing and sometimes the feeling of being attacked. The third group, which they called vestibular-motor, involves sensations of floating, flying or leaving the body.

Why the presence so often feels hostile is still debated. One widely discussed idea is that the brain's threat-detection systems, alarmed by the paralysis and the fear, generate a sense of a nearby agent, and dream imagery then gives that agent a shape. The chest pressure may come partly from the paralysis itself, since breathing continues automatically while the effort to take a deliberate deep breath meets muscles that will not respond. Both explanations fit the evidence well but are not fully proven.

In Newfoundland the experience has a name: the Old Hag, and someone who has it is said to have been hagged. The folklorist David Hufford, who began hearing such accounts during fieldwork there in the early 1970s, recognized that what people described matched what sleep researchers called sleep paralysis. His 1982 book The Terror That Comes in the Night argued that the experience has a stable core, the paralysis, the presence, the pressure and the fear, that turns up even in people who have never heard the local tradition. In his view the experience shapes the folklore at least as much as the folklore shapes the experience.

Names for the night visitor are everywhere. The English word nightmare originally referred not to a bad dream but to the mare, a spirit that sat on sleepers' chests; Scandinavian tradition knew a similar figure as the mara. German has Hexendrücken, "witch-pressing". In Japan the experience is kanashibari, "bound in metal", and a Chinese name, gui ya chuang, means "ghost pressing on the bed". In Brazil the Pisadeira is a crone who tramples the chests of people who go to bed on a full stomach.

The best-known image of all is Henry Fuseli's painting The Nightmare (1781), first shown at the Royal Academy in London in 1782 and now in the Detroit Institute of Arts. It shows a woman sprawled asleep with a demonic creature crouched on her chest and a horse's head pushing through the curtains behind her. It is still the image most often used to illustrate the experience.

More common than most people think. In 2011 Brian Sharpless and Jacques Barber reviewed 35 studies covering more than 36,000 people. They estimated that about 8 percent of the general population, 7.6 percent in their analysis, has had at least one episode in their lifetime. The figure was about 28 percent for students and about 32 percent for psychiatric patients, and higher still among people with panic disorder. Rates vary a good deal between studies and countries, partly because researchers ask the question in different ways.

A 2018 review by Dan Denis, Chris French and Alice Gregory, drawing on 42 studies, looked at what goes along with sleep paralysis. Episodes were associated with stress and traumatic experiences, being especially common in post-traumatic stress disorder, and with poor sleep quality and disrupted sleep, as well as with substance use, physical illness and some personality traits and beliefs. Many of these links come from surveys, so they show what tends to occur together rather than what causes what.

For most people sleep paralysis is rare: one or two episodes in a lifetime, often during a period of short or irregular sleep. A smaller group experiences it repeatedly, and researchers describe that as recurrent isolated sleep paralysis when there is no other sleep disorder behind it. The numbers matter for anyone weighing a bedroom encounter. An experience shared by millions of people with a well-described mechanism is the first possibility to consider, not the last.

Sleep paralysis itself is generally harmless. Episodes end on their own, and the paralysis does not damage the body, however frightening it feels. The fear is real, though, and some people start to dread going to sleep, which can make sleep worse.

Frequent episodes, or episodes that come with excessive daytime sleepiness, such as falling asleep unintentionally during the day, are worth raising with a doctor. Sleep paralysis is one of the recognized symptoms of narcolepsy, a neurological sleep disorder in which overwhelming daytime sleepiness is the central feature. People with narcolepsy may also have vivid hallucinations as they fall asleep and, in some cases, cataplexy, sudden muscle weakness triggered by strong emotions such as laughter. Sleep clinics can test for it.

For many people, simply learning that sleep paralysis is common and has a known mechanism takes away some of its terror, even if the experience itself feels the same the next time it happens.

A large share of night-visitor reports fit sleep paralysis closely: the figure at the end of the bed, the weight on the chest, the attempt to call out that produces no sound, all happening while the person was lying down at the edge of sleep. When an account includes those details, sleep paralysis is the explanation to rule out first, and it can be ruled in with surprising confidence.

It does not explain everything. It does not account for experiences that happen while a person is fully awake, standing and moving around, for several witnesses who see the same thing at the same time, or for physical traces left behind. Those need other explanations, or remain open.

Explaining the mechanism also does not settle what an experience means to the person who had it. Some people find the neuroscience a relief; others hold religious or spiritual interpretations that are theirs to keep. Hufford argued that researchers should take such accounts seriously as real experiences rather than dismissing the people who report them, and that is a good standard for believers and skeptics alike.