Sleep paralysis: the science behind the “ghost on your chest”

What is sleep paralysis?

Sleep paralysis is a brief spell in which a person is awake but cannot move or speak, while falling asleep or waking up. It happens when the muscle paralysis of rapid eye movement (REM) sleep, which normally keeps people from acting out dreams, continues into wakefulness. Eye movements and breathing carry on. A 2016 clinical review says episodes usually last from seconds to 20 minutes, with a mean of 6 minutes.

Sleep paralysis is real. It has been induced and recorded in sleep laboratories, and the NHS calls it scary but harmless, adding that most people have it only once or twice in their lives. The “ghost on your chest” belongs to the episode itself: a vivid hallucination, often of an intruder or of weight on the chest, in a state that mixes REM sleep with wakefulness.

Sleep paralysis examples: what people report

The NHS describes being awake but unable to move or speak, feeling that someone is in the room or that something is pushing you down, and feeling frightened. Researchers group the hallucinations into three kinds: intruder hallucinations, a sense of an evil presence; incubus hallucinations, pressure on the chest; and vestibular-motor hallucinations, illusory movement such as flying or leaving the body. The Sleep Foundation says about 75 percent of episodes include hallucinations. Episodes are most likely when the sleeper lies on their back, and while fear is typical, some people report positive episodes, especially with sensations of movement.

How the experiments worked

Few experiments have probed the neurophysiology of sleep paralysis, but two Japanese studies, summarized in a 2018 systematic review, induced it on purpose. In 1992 Takeuchi and colleagues studied 16 people who had had sleep paralysis at least twice and interrupted their sleep to trigger sleep-onset REM periods, REM that comes abnormally early, while polysomnography recorded them. A 2002 study of 13 people used a multiphasic sleep-wake schedule.

What they found

The 1992 study produced 6 episodes from 64 interruptions, and in all but one the sleeper reported hallucinations involving several senses, with fear and dread. The recordings showed abundant alpha brain waves, a rhythm of relaxed wakefulness not expected during sleep, while the muscle paralysis of REM sleep persisted. The 2002 study produced 8 episodes from 184 interruptions; 6 followed a sleep-onset REM period and none followed ordinary REM sleep.

Researchers have suggested that sleep paralysis is a dissociated state with both REM and waking brain activity. The paralysis is believed to come from the GABA and glycine systems that inhibit motor neurons during REM sleep. Why some people regain waking consciousness during REM sleep is still unknown.

The sleep paralysis demon explained

The “demon” fits these findings. The body stays paralyzed while perception of the real bedroom is clear, and the hallucinations of this mixed state are vivid, involve several senses and are often frightening. Intruder hallucinations supply the presence; incubus hallucinations supply the weight, and feelings of suffocation are common even though breathing continues. The 2016 review notes that sleep paralysis has been thought to feed beliefs in nighttime demonic attacks and alien abductions.

Interpretations vary by culture. Terms for sleep paralysis exist in more than 100 cultures, and in many places the experience is part of folklore. In Japan it is known as kanashibari. A 1978 paper interpreted the Old Hag phenomenon as sleep paralysis, a 2005 study of Khmer refugees was titled “The ghost pushes you down,” and episodes have been proposed as an explanation for accounts of witchcraft.

Replications and criticism

Surveys worldwide agree that sleep paralysis is common, but not on how common. A 2011 systematic review by Sharpless and Barber pooled 35 studies with 36,533 participants: 7.6 percent of the general population, 28.3 percent of students, 31.9 percent of psychiatric patients and 34.6 percent of people with panic disorder had had at least one episode. Single studies range from 2 to 60 percent; the Sleep Foundation cites about 20 percent and Cleveland Clinic about 30.

The 2018 review of 42 studies, by Dan Denis and colleagues, blamed part of the spread on the lack of a gold-standard measure and on question wording, which changes reported rates. Research relies on people recalling past episodes, and without longitudinal studies it cannot show whether anxiety brings on episodes or episodes raise anxiety. Both lab studies also counted REM as early if it began within 25 minutes of sleep onset; the American Academy of Sleep Medicine uses 15.

Who is more likely to have it

The 2018 review linked sleep paralysis with poor or disrupted sleep, stress and trauma, substance use, physical illness, anxiety symptoms and psychiatric disorders. A twin study estimated a moderate genetic influence, 53 percent, on whether people experience it. Shift workers reported it more often, 48 percent against 36 percent. Rates are particularly high in post-traumatic stress disorder: 65 to 100 percent of patients in American, Cambodian and Chinese samples, against 20 to 25 percent of healthy controls.

When to talk to a doctor

The NHS advises seeing a GP if you often have sleep paralysis and feel very anxious about going to sleep, or are very tired because you are not getting enough sleep. Cleveland Clinic says frequent episodes are worth discussing with a healthcare provider, since there may be a treatable underlying cause.

The Sleep Foundation suggests talking to a doctor about signs of narcolepsy: falling asleep without warning at inappropriate times, excessive daytime sleepiness or muscle weakness. Narcolepsy is a neurological disorder marked by excessive daytime sleepiness and cataplexy, sudden brief losses of muscle tone brought on by strong emotions such as laughter. Clinicians check for these features, sometimes with sleep tests, to tell it apart from isolated sleep paralysis.

How it is seen today

Sleep medicine classifies the condition: the International Classification of Sleep Disorders includes recurrent isolated sleep paralysis, defined by repeated episodes that cause clinically significant distress. Treatment evidence is thin; as of 2016 not one randomized controlled trial had been run. The NHS suggests 7 to 9 hours of sleep, a regular bedtime and not sleeping on your back, and says a doctor may offer cognitive behavioral therapy or a medicine usually used to treat depression.

Real or Legend files sleep paralysis as confirmed. The stamp covers the phenomenon and the core of its explanation: episodes have been induced and recorded in the laboratory, and surveys of more than 36,000 people show they are common. It does not cover the supernatural reading. The presence and the weight on the chest are hallucinations of a mixed sleep-wake state, however real they feel.

Questions people ask