What is sleep paralysis, and why do people sometimes sense a presence in the dark?

What do people observe?

Sleep paralysis is a short spell when a person cannot move their arms, legs or body, yet their eyes can still move, their breathing goes on, and their mind is clear. It comes while falling asleep or waking up, and it is more likely when people sleep on their backs. A 2011 review pooled 35 studies covering 36,533 people. At least one episode had happened to 7.6% of the general population, 28.3% of students and 31.9% of psychiatric patients.

What makes it memorable is what comes with it. The 2011 review says vivid hallucinations, seeing, hearing or feeling things that are not there, often come with episodes. The review notes that common themes include strange movements such as floating, seeing one’s own body, and menacing intruders in the bedroom. Fear is common too. In one study it summarizes, 90% of a student group and 98% of an online group reported fear during episodes.

Rates vary from place to place. In a 2013 study, 25% of people surveyed in Denmark had had sleep paralysis, compared with 44% in Egypt. Egyptians who had it also reported about three times as many episodes in their lives.

What do people infer from it?

For much of history, people have explained these nights as a visit. The 2011 review lists witchcraft, evil spirits and extraterrestrials among the explanations different cultures have given. In a 2014 study in Egypt, 48% of people from the general population of Cairo who had sleep paralysis believed the jinn caused it. Students at the American University in Cairo were less likely to give a supernatural cause, and people in Denmark less likely still.

A 2005 study of 100 Cambodian refugees who were psychiatric patients took its title from the phrase “the ghost pushes you down.” In that group, 42% had had sleep paralysis in the past year. Of those, 91% saw a being approach them, and all of them had panic attacks. It was far more common in patients with post-traumatic stress disorder (PTSD). The researchers wrote that cultural ideas about ghost and sorcery attacks, and links to past trauma, seemed to heighten the panic greatly.

These beliefs are not strange or foolish. They are reasonable ways to make sense of something that feels like an attack by someone else. The question is what the evidence says about the cause.

Does the evidence support a visitor?

The evidence points to the brain at the edge of sleep. During REM sleep, the body stays still. MedlinePlus explains that if you wake suddenly from REM, your brain is awake but your body is still in REM mode and cannot move. The 2011 review says sleep studies trace the episodes to REM activity that carries over into the shift between sleep and waking.

That also helps explain the visions. Vivid dreaming is most common in REM sleep, and during sleep paralysis, dream-like sensations or hallucinations can appear while the person is aware of the room. (MedlinePlus says this happens in rare cases; the 2011 review says such hallucinations often accompany episodes.) A 1999 study by the Waterloo team found three clusters of experiences. One, the “Intruder,” combines a sensed presence, fear, and sounds or sights. They proposed that it starts in an alert, threat-watching state set off in the midbrain, deep in the brain. Another, the “Incubus,” combines chest pressure, trouble breathing and pain, which they traced to the way the muscle shutdown of REM sleep changes how breathing feels. A third brings floating, out-of-body feelings and sometimes bliss.

The sense of someone nearby can also be made in the lab. In a 2006 case, a patient was being tested before epilepsy surgery. Electrical stimulation of one spot on the left side of the brain made the patient feel a “shadow person” nearby, one that copied the patient’s body position. In a 2014 study, a team including researchers in Switzerland used a robot that scrambled the link between people’s own movements and what they felt. Healthy volunteers then felt that someone was nearby. The team concluded that this feeling comes from the brain misreading signals from the person’s own body. These studies did not look at sleep paralysis itself. But they show that a healthy brain can produce a convincing “someone is here” from the inside.

Why do we feel watched, and why is the dark scary?

In a 2013 study, people judged which way computer-made faces were looking. When the eyes were hard to see, everyone tended to judge that the gaze was aimed at them. The researchers say this expectation takes over when things are uncertain, such as at night. A dark bedroom is exactly that kind of place.

Can people sense a stare from behind? In a 2000 test, people did a little better than chance at guessing when they were being stared at, but only when they got feedback and the sequence of stares was not truly random. With truly random sequences, the effect vanished. The authors concluded that people had most likely picked up the pattern in the sequences without knowing it. On the other side, Jessica Utts’s 1995 review of government-funded psychic research reported two experiments at the company SAIC. A person sat in a room with a video camera on them while their skin response, a measure of the body’s arousal, was recorded. In a distant room, an observer saw the person on a computer monitor at random times and tried to influence them. In both experiments, skin response was higher during the watched periods than during control periods. Utts wrote that the result raises substantially more questions than it answers, since the mechanism is unknown.

Fear of the dark has a body side too. In a 1997 study at Yale, darkness made people’s startle reflex, the jump at a sudden loud sound, significantly stronger. The boost was bigger in people who remembered being more afraid of the dark as children. The researchers suggested that darkness changes our emotional state rather than simply making us pay more attention.

What is the strongest objection, and what is unresolved?

The strongest objection is that the presence feels so real, and that people in many cultures describe the same kind of figure. Both points are true. But the Waterloo team wrote that these accounts are also consistent with what is known about REM sleep. A shared human brain would be expected to produce similar experiences everywhere, dressed in each culture’s own stories.

Some questions are still open. MedlinePlus says the exact cause of sleep paralysis is not fully known. It has been linked to too little sleep, irregular schedules, stress, sleeping on one’s back, narcolepsy, and conditions such as PTSD and panic disorder. Nobody knows exactly why rates differ so much between countries. In Egypt, believing in a supernatural cause went along with more fear and longer spells of not being able to move. That link does not show which came first.

Whether spirits exist at all is a separate question that this page does not settle. Its claim is narrower: the frightening visitor of sleep paralysis has a well-studied explanation in the sleeping brain. For hauntings and other ghostly experiences, see Are ghosts real? For claims about minds sensing things at a distance, see Do psychic abilities hold up when tested?

What helps, and how can you check it yourself?

• Protect your sleep. The NHS suggests 7 to 9 hours of sleep, regular bed and wake times, and exercise, but not in the last hour before bed. It advises against big meals, smoking, alcohol or caffeine shortly before bed, and against sleeping on your back.
• Know when to get help. The NHS says to see a doctor if episodes are frequent and you feel very anxious about sleep or tired all the time. MedlinePlus notes that sleep paralysis can be a sign of narcolepsy. Treatment can include cognitive behavioural therapy (CBT) or a low dose of a medicine usually used for depression.
• Keep a sleep diary. Note the time, how much you slept, your sleeping position and your stress level. Patterns may show up.
• Test the visitor. If a figure seems to tell you something, write it down before anyone checks it. Could it have been known, guessed or looked up?
• Remember what it is. Spells end on their own, MedlinePlus says, or when someone touches or moves you.

The answer stays the same after all this detail. Sleep paralysis is common, real and harmless, even when it is terrifying. The evidence points to a brain caught between dreaming and waking as the source of the presence in the dark.