Sleep Paralysis: What It Is and Why It Happens

Sleep paralysis is a temporary inability to move or speak that occurs at the boundary between sleep and wakefulness — when falling asleep or just after waking. The person is fully conscious but cannot move their limbs, speak, or in most cases even open their eyes. Episodes frequently include vivid hallucinations and an overwhelming sense of dread. While usually lasting only seconds to a few minutes, the experience can be profoundly frightening — and has shaped supernatural folklore across cultures for centuries.

What Is Sleep Paralysis?

To understand sleep paralysis, it helps to understand what happens during normal REM sleep. During REM (rapid eye movement) sleep — the stage most associated with vivid dreaming — the brain actively suppresses movement in the body. This state is called REM atonia and it is protective: it stops us from physically acting out our dreams. The brain generates the paralysis before the dreams begin and maintains it throughout REM sleep.

Sleep paralysis occurs when this atonia persists — or begins prematurely — while the person's conscious awareness returns. The brain is caught between states: the body is still in "sleep paralysis mode" from REM, while the mind is awake or becoming awake. The result is a fully conscious person who cannot move.

There are two types based on timing:

• Hypnagogic sleep paralysis: Occurs as you are falling asleep — the paralysis begins before REM sleep is fully established
• Hypnopompic sleep paralysis: Occurs upon waking from REM sleep — the paralysis persists briefly after consciousness returns; this is the more common form

How Common Is Sleep Paralysis?

Sleep paralysis is far more common than many people realise. Population studies suggest that around 8% of people experience it at some point in their lives. In certain groups, the prevalence is much higher — approximately 28% of students and 31.9% of psychiatric patients in one meta-analysis. Among people with narcolepsy, sleep paralysis is a defining feature occurring in the majority of patients.

Most people who experience sleep paralysis do so only occasionally — perhaps once or twice in a lifetime. A smaller group experiences it recurrently, and for them it can become a significant source of anxiety, particularly about going to sleep.

What Does Sleep Paralysis Feel Like?

The core experience is a sudden awareness of being awake but completely unable to move — not "too tired to move" but physically locked. Attempting to call out for help or move a limb produces no result. The chest may feel pressed down or constricted, making breathing feel difficult (though breathing continues normally — the sensation of pressure comes from a heightened awareness of respiratory effort).

In many episodes, hallucinations are present. These are categorised into three types:

• Intruder hallucinations: The terrifying sensation that a threatening presence is in the room — someone watching, approaching, or standing nearby. The person cannot see or fully identify the presence but feels it intensely.
• Incubus hallucinations: A sensation of pressure on the chest, often combined with the feeling of being pinned down or choked. The name comes from the medieval legend of a demon (incubus) that would sit on sleeping people's chests — a direct cultural reflection of this experience.
• Vestibular-motor hallucinations: Sensations of movement — floating, flying, spinning, falling, or leaving the body. These can be frightening or, for some, pleasant.

These hallucinations are neurological in origin — they arise from the same brain processes that generate REM dreams — and are not a sign of mental illness or psychosis. However, they can be so vivid and realistic that people struggle to believe they were not real, and many interpret them through cultural or religious frameworks (alien abduction, demonic attack, supernatural visitation).

What Triggers Sleep Paralysis?

Several factors are associated with increased frequency of sleep paralysis episodes:

• Sleep deprivation and disrupted sleep schedules: The most consistently reported trigger. Any disruption to the regularity or quantity of sleep increases the likelihood of REM intrusion into wakefulness.
• Sleeping on the back (supine position): Sleep paralysis occurs significantly more often when sleeping supine, possibly because this position promotes REM sleep stages or makes transitions between stages more unstable.
• Stress and anxiety: High psychological stress disrupts sleep architecture and is associated with more frequent episodes.
• Shift work and jet lag: Any circadian rhythm disruption that forces sleep at unusual times can trigger episodes.
• Narcolepsy: Sleep paralysis is a defining symptom of narcolepsy and occurs with significantly greater frequency in affected individuals.
• Certain medications: Some antidepressants, particularly SSRIs and SNRIs that suppress REM sleep, can paradoxically cause REM rebound when doses are missed, potentially triggering sleep paralysis.
• Substance use: Alcohol disrupts REM sleep; rebound on withdrawal nights is associated with more vivid REM experiences including sleep paralysis.

Across Cultures and History

Sleep paralysis has left a striking fingerprint on human folklore across time and geography. The medieval "Old Hag" of Newfoundland folklore, the "Kanashibari" of Japan (being "bound in metal"), the Arabic "Jathoom," the Turkish "Karabasan," the South African "Tokoloshe" — all describe a remarkably similar experience: a terrifying presence that pins the sleeper down and prevents movement. The incubus and succubus demons of medieval Europe almost certainly draw from sleep paralysis accounts.

Modern studies suggest that sleep paralysis, particularly the intruder and incubus hallucinations, may account for a significant proportion of reported paranormal experiences and alien abduction accounts. Understanding the neurological basis removes the mystery without diminishing how genuinely terrifying the experience is.

How to Reduce Sleep Paralysis Episodes

For most people, addressing the common triggers produces a significant reduction in frequency:

• Prioritise consistent, adequate sleep — 7–9 hours per night on a stable schedule
• Avoid sleeping on your back; use a body pillow or sew a tennis ball into the back of your pyjama top to encourage side-sleeping
• Manage stress through regular exercise, relaxation techniques, or therapy
• Maintain a consistent wake time, even on weekends
• Limit alcohol, particularly close to bedtime
• For people with frequent, distressing episodes, low-dose antidepressants that suppress REM sleep (such as clomipramine or fluoxetine) can be considered under medical supervision

What to Do During an Episode

Knowing what sleep paralysis is significantly reduces its psychological impact. During an episode:

• Remind yourself that what you are experiencing is sleep paralysis — it is temporary, harmless, and will pass
• Attempt to move a small muscle group — wiggling a finger or toe — rather than trying to move the whole body at once
• Focus on your breathing and try to breathe slowly and deliberately
• Some people find that trying to move their eyes from side to side can disrupt the paralysis
• Do not try to "fight" the sensation aggressively — this tends to increase panic without ending the episode faster

References

• Sharpless BA, Barber JP. Lifetime prevalence rates of sleep paralysis. Sleep Medicine Reviews. 2011;15(5):311–315.
• Cheyne JA, Rueffer SD, Newby-Clark IR. Hypnagogic and hypnopompic hallucinations during sleep paralysis. Consciousness and Cognition. 1999;8(3):319–337.
• Jalal B. The neuropharmacology of sleep paralysis hallucinations. Psychopharmacology. 2018;235(11):3083–3091.
• American Academy of Sleep Medicine. International Classification of Sleep Disorders, 3rd edition. 2014.