Sleep paralysis is a brief state, while falling asleep or waking up, in which a person is conscious but cannot move or speak. It happens when the muscle shutdown of REM (dream) sleep, called REM atonia, continues for a short time after the mind has woken. It is often accompanied by vivid hallucinations, such as a figure in the room or a weight on the chest.
It matters because the experience is common, frightening and old. The same scene, lying awake and unable to move while something sits on the chest, lies behind the Old Hag of Newfoundland, the mare that gave English the word "nightmare", the medieval incubus and, some researchers argue, part of the modern alien-abduction story. Knowing how it works helps sleepers stay calm and helps readers judge what those traditions may be describing.
What happens in the body during sleep paralysis
Sleep paralysis happens when two parts of sleep fall out of step: the mind becomes aware while the body is still in REM atonia. During REM sleep, the stage most strongly linked to vivid dreaming, the brain switches off the body's voluntary muscles. This protective feature stops people acting out their dreams.
In sleep paralysis the person regains awareness of the room while the muscles are still switched off. They can usually open their eyes, but cannot lift an arm, turn over or call out. The muscles that keep a person alive are not affected: breathing continues automatically and the eyes can still move. The common feeling of being unable to breathe comes from fear and the sense of pressure, not from the breathing muscles failing.
Episodes usually last from a few seconds to a few minutes, though longer ones are occasionally reported. They end on their own, often when the person manages a small movement, is touched or spoken to, or simply wakes fully. Sleep paralysis is not known to cause physical harm, however distressing it feels.
Sleep scientists call episodes in people with no underlying sleep disorder "isolated" sleep paralysis, as distinct from sleep paralysis that is part of narcolepsy. When isolated episodes keep returning, recurrent isolated sleep paralysis is recognised as a sleep-wake disorder in its own right.
Why sleep paralysis comes with hallucinations
The hallucinations happen because the dreaming brain is still partly running while the person is awake enough to see the bedroom. Dream imagery and sensations are laid over the real room, which is why it feels so real.
Hallucinations while falling asleep are called hypnagogic, a term the French scholar Alfred Maury introduced in 1848. Those that happen while waking up are called hypnopompic, a word coined by the psychical researcher Frederic Myers in 1904.
The content is strikingly consistent. In a 2003 paper in the journal Dreaming, psychologist J. Allan Cheyne of the University of Waterloo grouped these hallucinations into three clusters:
- The Intruder: a sensed presence, usually felt to be hostile, often with footsteps, whispers, or a dark shape in the doorway or at the foot of the bed.
- The Incubus: pressure on the chest, difficulty breathing, and a feeling of being held down, choked or attacked.
- Vestibular-motor experiences: floating, spinning, falling, or the sense of leaving the body.
The first two tend to appear together. Researchers link them to the brain's threat-detection systems, which stay active during REM sleep: the mind senses danger while the body cannot respond, and fills in a threat, a figure in the room or a weight on the chest. The floating and out-of-body cluster is less tied to that threat response and overlaps with the experiences discussed in our guide to lucid dreaming and astral projection.
How common is sleep paralysis
Sleep paralysis is common, but less common than some online claims suggest. The most useful figure comes from a systematic review by Brian Sharpless and Jacques Barber, published in Sleep Medicine Reviews in 2011. It pooled 35 studies covering 36,533 people and found a lifetime rate of about 7.6% in the general population. In other words, about one person in thirteen has had at least one episode.
The rates were much higher in two groups. About 28.3% of students and 31.9% of psychiatric patients reported at least one episode. Students often sleep irregularly and too little, which fits the known risk factors. A higher rate in a group is an association, not a sign that an individual episode means mental illness. Much higher figures circulate online, usually from single surveys of particular groups; the pooled 7.6% is the more reliable guide.
Shadow people, the Old Hag and the night-mare across cultures
The phrase "shadow people" is a modern name for the Intruder hallucination: dark, often featureless figures seen standing in the room during an episode. Many cultures have a name for the experience, and many describe something sitting on the chest.
Folklorist David Hufford explored this in The Terror That Comes in the Night: An Experience-Centered Study of Supernatural Assault Traditions (University of Pennsylvania Press, 1982). His starting point was the "Old Hag" tradition of Newfoundland, where someone who woke paralysed with a crushing weight on the chest was said to be "hag-ridden". Hufford argued that a real, recurring experience lies beneath supernatural-assault traditions across cultures, rather than the traditions simply producing the experience.
The parallels are wide:
- In Japan, the experience is called kanashibari, literally "bound or fastened in metal".
- In China, it is known as guǐ yā chuáng, "ghost pressing on the bed".
- In Turkey, the karabasan, "the dark presser", is said to sit on the sleeper's chest and take their breath.
- In Brazil, the pisadeira, "she who steps", is a thin old woman who treads on the chests of people who go to bed on a full stomach.
- In Old English, the mare was a spirit that pressed on sleepers. "Nightmare" referred to this experience before it took on its modern general meaning of a bad dream.
- In medieval Europe, incubi and succubi were thought to sit on or lie with sleepers at night.
The best-known image of the experience is Henry Fuseli's painting The Nightmare (1781), now in the Detroit Institute of Arts. It shows a sleeping woman with a demonic, ape-like incubus crouched on her chest and a horse's head pushing through the curtains.
Some modern readers also connect sleep paralysis to the night demons of ancient Mesopotamia, such as the lilītu, whose story is traced in our article on Lilith from Mesopotamian demons to Adam's first wife. That connection is drawn by later interpreters: the ancient texts describe night demons, not sleep paralysis as such.
Is sleep paralysis linked to alien abductions
Some researchers argue that many alien-abduction reports begin as sleep-paralysis episodes. In a New Scientist article published on 19 November 1994, psychologist Susan Blackmore proposed sleep paralysis as a possible "core event" in abduction stories: waking unable to move, sensing a presence, and feeling pressure on the body. She suggested that alien abductions may be a modern version of older sleep-paralysis myths such as the incubus and the Old Hag.
Harvard psychologists Richard McNally and Susan Clancy tested a version of this idea. In a 2005 paper in Transcultural Psychiatry, they studied ten people who reported being abducted and found that their accounts matched sleep paralysis with hypnopompic hallucinations, which the abductees had interpreted as aliens. The authors concluded that people tend to explain such episodes using "personally plausible cultural narratives". A Newfoundlander of an earlier generation might see the Old Hag, a medieval sleeper an incubus, and a late-20th-century American a grey alien.
This explanation does not cover every abduction claim. The best-known case, the Betty and Barney Hill abduction, began as a night-time drive on a New Hampshire road, with details later recovered under hypnosis. It does not follow the sleep-paralysis pattern, so it needs to be judged on its own evidence.
What makes sleep paralysis more likely
Anything that disrupts sleep makes sleep paralysis more likely. The factors most consistently linked to it are:
- Sleep deprivation and not getting enough sleep.
- Irregular sleep schedules, including frequent changes in bedtime and wake time.
- Psychological stress and anxiety.
- Sleeping on the back, which many people who have episodes report.
- Alcohol use.
- Other sleep disorders, including narcolepsy and obstructive sleep apnea.
How people reduce sleep paralysis and get through an episode
The most common advice is to protect sleep itself: go to bed and get up at regular times, get enough sleep, sleep on the side rather than the back, limit alcohol and avoid caffeine late in the day. Stress management, such as winding down before bed, is also widely recommended. For more on steady sleep habits, see the 4biddenknowledge guide to the science of sleep and getting your best rest.
For the episode itself, common self-help advice includes:
- Focus on moving one small part of the body, such as a finger or a toe, which often breaks the episode.
- Slow the breathing deliberately rather than fighting for air. Breathing is still working.
- Remind yourself what is happening: it is a known sleep state, it is brief, and it passes.
Many people find that knowing the explanation makes episodes less frightening. Evidence for specific approaches is limited. In a 2016 clinician's guide to recurrent isolated sleep paralysis, Sharpless noted that the condition is under-recognised and that proposed treatments, while promising, still need larger, well-controlled trials.
Anyone whose episodes are frequent or distressing, interfere with sleep or daily life, or come with excessive daytime sleepiness or sudden muscle weakness while awake (possible signs of narcolepsy) should consult a clinician or a sleep specialist.
Billy Carson's reading
Billy Carson has opened this question to his audience directly. His podcast-network channel streamed a live session titled "Sleep Paralysis: Demons, Aliens or Just A Dream?", which runs about an hour and twenty-six minutes. The stream's description sets out the three readings people give the experience. It notes that cultures have described it as everything from demonic attack to alien abduction. It also says there is no scientific evidence supporting extraterrestrial involvement, describes the REM-sleep explanation, and advises speaking with a doctor if episodes become frequent.
That framing takes the cross-cultural reports seriously as human experience while pointing to a well-supported scientific mechanism. Carson's wider interest in sleep and altered states runs through the lucid dreaming and astral projection pillar, and his show Beyond the Known on 4BK TV covers paranormal case files.
Watch, read and go deeper
The full live stream, "Sleep Paralysis: Demons, Aliens or Just A Dream?", is embedded with this article and is the best place to hear Billy Carson and his audience work through the question; for more case files, watch Beyond the Known on 4BK TV. To see how dream science and older dream traditions read the images of sleep, continue with dream interpretation from Freud and Jung to sleep science. The Consciousness topic hub brings together every 4biddenknowledge guide on the mind, sleep and states of awareness.
Frequently asked questions
Can sleep paralysis hurt you?
Sleep paralysis is not known to cause physical harm. Breathing continues automatically, even when it feels restricted, and episodes end on their own, usually within seconds to a few minutes. Frequent episodes can disturb sleep, which is a reason to talk to a clinician.
Why do people see shadow figures during sleep paralysis?
Shadow figures are a form of the Intruder hallucination described by J. Allan Cheyne. The brain's threat-detection systems are active while the body cannot move, so the mind fills in a source for the danger, often a dark figure in the doorway or at the bedside.
How common is sleep paralysis?
A 2011 systematic review by Brian Sharpless and Jacques Barber, covering 35 studies and 36,533 people, found that about 7.6% of the general population have had at least one episode. Rates were higher among students (28.3%) and psychiatric patients (31.9%).
What is the Old Hag?
The Old Hag is the Newfoundland name for the experience of waking paralysed with a crushing weight on the chest; a person who had it was said to be "hag-ridden". Folklorist David Hufford studied the tradition in The Terror That Comes in the Night (1982). He argued that it reflects the same experience found in supernatural-assault traditions in many other cultures.
When should someone see a doctor about sleep paralysis?
Occasional episodes are common and usually linked to poor or irregular sleep. Someone should consult a clinician if episodes are frequent or distressing, affect daily life, or come with excessive daytime sleepiness or sudden muscle weakness while awake. Those can be signs of narcolepsy or another sleep disorder that a sleep specialist can assess.
Sources and further reading
- Brian A. Sharpless and Jacques Barber, "Lifetime prevalence rates of sleep paralysis: a systematic review," Sleep Medicine Reviews (2011)
- Brian A. Sharpless, "A clinician's guide to recurrent isolated sleep paralysis," Neuropsychiatric Disease and Treatment (2016)
- J. Allan Cheyne, "Sleep paralysis and the structure of waking-nightmare hallucinations," Dreaming (2003)
- David J. Hufford, The Terror That Comes in the Night: An Experience-Centered Study of Supernatural Assault Traditions (University of Pennsylvania Press, 1982)
- Susan Blackmore, article on alien abduction and sleep paralysis, New Scientist (19 November 1994)
- Richard J. McNally and Susan A. Clancy, "Sleep Paralysis, Sexual Abuse, and Space Alien Abduction," Transcultural Psychiatry (2005)
- Henry Fuseli, The Nightmare (1781), Detroit Institute of Arts
This article is educational and is not medical advice; anyone with frequent or distressing sleep paralysis should consult a qualified clinician.
