It has been the subject of documentaries, horror stories, and even internet memes—but what do we really know about sleep paralysis, a potentially terrifying phenomenon that affects nearly 10% of the population?
At one time, the phenomenon was thought to be caused by demons squatting on the chests of sleepers, and a description of it helped to give us our modern word nightmare, which we use for any bad dream. Even today, sleep paralysis is poorly understood, its causes and pathophysiology a matter of theory and conjecture, rather than settled medical fact.
Just as we don’t know why sleep paralysis affects certain individuals—roughly 7.6% of the population, according to data from the National Institutes of Health—we also don’t know why episodes of sleep paralysis sometimes recur, while other times are isolated incidents.
What is Sleep Paralysis?
As one is just waking up or falling asleep, one may find oneself caught in a sort of limbo between the two states—conscious, but unable to move. During this experience, one may suffer from hallucinations, including auditory, visual, and tactile sensations of things that are not actually there, such as voices or sounds, or even shadowy intruders in your room. This sensation of not being alone can induce inexplicable terror. Some have even reported feeling the sensation of being dragged out of bed or flying through the air. This state is sleep paralysis.
One anonymous reader of The Lineup says, "I always know a sleep paralysis episode is starting when I wake up and my whole body is vibrating. Sometimes I hear strange, indecipherable, distorted voices. I see figures dart past me in the corner of my eyes, though I can never get a good look at them. I always feel the sense of a menacing presence, as though I’m not alone in my room. I also hallucinate—for example, I will see that my door is open when it’s not. Other times, I've been convinced I’ve gotten up out of my bed—only to discover I’m still laying down. It’s horribly scary and disorienting."
For another reader, her experiences with sleep paralysis are extremely vivid and intense. She often sees full-fledged people—or creatures—crouching at the foot of her bed, watching her. In one instance, she saw the witch from The Conjuring hovering over her bed. And those are the good days. Her experiences with this phenomenon get more intense when she feels the sensation of someone stroking her hair or squeezing her hand. Most of the time, she reports, she's absolutely terrified—these instances throw a wrench into a sleep cycle that’s already difficult due to chronic insomnia. But sometimes she’s aware enough of the situation that even the scariest of hallucinations feel like old friends she just has to wait out the paralysis with. This reader also divulged that her brother struggles with sleep paralysis as well.
Sleep paralysis is often conflated with what is known as a “night terror” or “sleep terror,” in part because the hallucinations experienced during sleep paralysis—combined with the helplessness that comes with being unable to move or speak—can be quite terror-inducing on their own. However, night terrors are a particular class of parasomnia, quite separate from sleep paralysis, and it is possible to experience one without the other, or both together in the same night.
Though night terrors superficially resemble nightmares or bad dreams, they actually occur during non-rapid eye movement (NREM) sleep, according to the Mayo Clinic. During NREM sleep, a person's muscles are "turned off", so to speak—so that when they dream, they aren't moving all over the place. This is why a person who becomes conscious while still in NREM sleep will not be able to move.
A person experiencing night terrors may appear to be awake, but are often unaware of their actions, and may have no memory of them when they do wake up. The effects of a night terror have been described as similar to a panic attack, and the phenomena may affect as many as 40% of children, though it is considerably less common in adults.
The Origins of Sleep Paralysis
Though the term “sleep paralysis” was coined in 1928 by British neurologist S. A. K. Wilson, the phenomenon itself is as old as sleep. In past centuries, such conditions were often attributed to demons or evil spirits, who would squat upon the chests of victims—resulting in shortness of breath, one of the many symptoms associated with sleep paralysis.
In Old English, these evil spirits were known as “mare” or “maere,” which led Samuel Johnson, in his 1755 Dictionary of the English Language, to describe parasomnias such as sleep paralysis or night terrors as “nightmares,” which is still the word we use for any vividly bad dream.
These depictions of bad dreams as evil spirits has inspired countless artists over the years, with perhaps the most famous image ever associated with bad dreams being Henry Fuseli’s 1781 masterpiece The Nightmares, which depicts a sleeping woman being preyed upon by dark figures. It is an image that has been repurposed and reproduced countless times throughout the ages—and one that may feel uncannily familiar to those who suffer from sleep paralysis.
Although sleep paralysis has been observed all over the world, the specifics of symptoms and hallucinations seem to vary by cultural context, leading some researchers to suggest that cultural priming may actually increase or decrease the likelihood of sleep paralysis among members of that culture. Other researchers, meanwhile, have used the vivid and lifelike hallucinations experienced by those who suffer from sleep paralysis to explain a variety of seemingly paranormal phenomena, ranging from hauntings to alien abductions.
What Causes Sleep Paralysis?
That may be the scariest part: we don’t really know. There are currently several major theories suggesting possible causes, or partial causes, for sleep paralysis, but the ultimate mechanism remains unknown.
One of the leading theories operates from the understanding that sleep paralysis seems to occur when REM sleep overlaps with the waking stages of sleep. Pursuing this line of reasoning, studies have found that individuals who experience sleep paralysis may have shorter periods of REM and NREM sleep cycles, which can lead to what is known as “fragmentation” of REM sleep. Such fragmentation can have a variety of effects on the quality of sleep, and often occurs when sleep patterns are disrupted – which has been shown to possibly precipitate instances of sleep paralysis.
Another theory tackles the problem from a similar direction, but with slightly different assumptions. Instead of focusing on the length of sleep cycles, it hones in on the neural functions that regulate sleep and wakefulness, suggesting that the cells that would normally send out the signals to wake us up might be “underactivated” in certain individuals, meaning that they have difficulty in overcoming the neurological signals that otherwise keep us asleep. This is partly supported by an overlap between instances of sleep paralysis and other sleep disorders, including narcolepsy.
Research has also found potential genetic components in determining whether an individual is likely to experience sleep paralysis. Studies conducted on identical twins have found that if one experiences sleep paralysis, the other is highly likely to do so as well. The exact nature of this genetic component remains unknown, however.
It is very likely that all of these factors play some role in determining whether or not an individual will suffer from sleep paralysis, even while cultural factors may affect what sorts of stimuli an individual experiences during an episode of sleep paralysis. For now, however, the phenomenon remains mysterious—and its effects literally petrifying.
Diagnosis of sleep paralysis is reliant partly on the reporting of the individual and must be distinguished from a variety of related phenomena, including night terrors, PTSD, and the terrifyingly-named “exploding head syndrome,” or EHS. Even once a diagnosis has been made, there are few treatments. Medications are rarely prescribed for sleep paralysis, and none have yet been found that completely prevent the phenomenon. Instead, most sufferers rely on changes to sleep hygiene or cognitive-behavior therapy. And, in the meantime, just get used to the horrifying idea of occasional waking nightmares.
A telling documentary-horror film that reveals the—if not scientific, but psychological—impacts of sleep paralysis is The Nightmare. This chilling film interviews real people afflicted by this terrifying phenomenon, but it heightens the horror by creatively enacting the experience for the viewer. This film does a pretty outstanding job portraying what it feels like to actually experience a sleep paralysis episode. It's available to stream via Amazon Prime video or a Shudder subscription.