It’s known as “Ghost Depression” in China, “Kanashibari” in Japan, meaning to be bound or fastened by metal strips, and “Karabasan” or ‘The Dark Presser’ in Turkey. The latter sounds oddly like a 1980s metal band, but these three terms all refer to the same thing – the often terrifying and little understood ordeal of sleep paralysis, which is believed to have left various imprints on our culture throughout the millennia, from tales of ghosts in the night to visits from aliens.

Over the past few months, sleep paralysis has made its way to the big screen for the first time in the shape of new docu-drama The Nightmare. With the help of a variety of special effects, director Rodney Ascher brings to life the often terrifying bedtime experiences suffered by individuals around the world. And they’re more common than you might think. Studies suggest that around 8% of the general population, 28% of students and 32% of psychiatric patients have experienced sleep paralysis at least once.

“It’s happened to me quite a few times,” says Santi, a 25-year-old civil engineer. “It feels like you’re awake but at the same time you know you’re asleep. Some things seem a bit weird and you can’t move. Some people say that it feels like somebody’s pinning you down. I had one experience when I could see the landing of the stairs from my bed and I had this feeling there was a big, black dog there. And because you’re paralysed you often feel quite paranoid. Another time, I remember feeling convinced that someone was coming to get me and I needed to wake up as soon as possible. But I couldn’t.”

Santi describes his experiences of sleep paralysis as distinctly unpleasant and comfortable, but compared to other anecdotes, they’re relatively mild.

“I had one patient who was lying in bed and woke up to see a little vampire girl with blood coming out of her mouth,” says Brian Sharpless, a clinical psychologist at Washington State University and author of the book, Sleep Paralysis: Historical, Psychological, and Medical Perspectives. “This is an example of a really vivid, multi-sensory hallucination. She could feel this vampire figure grabbing onto her arms, pulling her, and saying she was going to drag her to hell and do all these terrible things to her.”

But what exactly is sleep paralysis, and why does it occur?

Sleep may appear a very natural part of our existence, but in reality it’s a highly complex and subtle neurological process with a lot of room for stuff to go wrong, both as we’re going to sleep and as we’re waking up. Things can misfire due to a variety of reasons ranging from jetlag, unusual sleep patterns, anxiety, trauma, to alcohol or other substances which suppress the period known as REM or deep sleep. Suppressing REM means that instead of this period of the sleep cycle occurring at the beginning of the night, it rebounds towards the end as your brain tries to make up for the lost REM time. But this can provide the trigger for an odd sequence of events.

REM is when we experience our most vivid dreams and during this spell, the brain sends your body into a state of complete paralysis. This is a perfectly normal event which occurs every night and we believe it’s a mechanism to prevent us from acting out our dreams which could be highly dangerous. But when sleep goes wrong, you can actually wake up during the REM period, while your body is still paralysed.

Many people who suffer from these abnormal occurrences only experience paralysis. But because they are both awake and still in the REM stage of sleep, some will begin hallucinating with their eyes open, projecting vivid and often threatening dreams into their bedroom surroundings.

“Imagine this scenario where you wake up and find you’re paralysed,” says Baland Jalal, a neuroscientist at the University of California. “Naturally you panic and try to move and so your motor cortex in the brain starts firing and sending all these signals to your limbs. But nothing’s coming back because you’re trapped in this temporary state, and so your brain has no perceptive feedback to create a sense of what your body looks like. This leads to a distortion of your sense of self, and so you might have an out of body experience, or you might see various shapes appear which are actually disfigured versions of you.”

Trapped in this semi-wakened state, with shadowy figures filling the room, anxiety levels understandably peak. “You have this vague sense that there’s something in the room with you,” Sharpless says. “You feel a bit like a prey animal. It’s like walking alone through Soho late at night: you sense that someone might be watching you.”

It’s a terrifying situation, and as your confused brain desperately tries to interpret the array of signals it’s receiving, it can insert cultural beliefs or memories into the situation. “Adding original features, scenarios or stories to try and make sense of what you’re experiencing is a very human thing to do,” Jalal says. “And this is why people see ghosts, demons, aliens or even figments from their past appearing to attack them.”

It’s very common to work memories or adaptations of past events into ordinary dreaming, especially when they have powerful emotional connections, something Sigmund Freud called “the day’s residue” of the dream. But when this coincides with sleep paralysis, the consequences can be highly disturbing.

One study from Harvard University looked at Cambodian refugees who fled the Khmer Rouge and subsequently suffered from sleep paralysis many years later. Many found themselves vividly reliving their experiences through their hallucinations. Sharpless is currently treating a group of college students whose first sleep paralysis episode occurred after experiencing the death of a grandparent. “Because that person is very much in their thoughts, their mind has often worked the dead relative into the scenario which is really traumatic,” he says.

In many cultures, humanity’s attempts over the centuries to seek explanations, have led to deep- held superstitions about witches and dark magic. Such fairytales act as a primer for the hallucinations.

“It creates a positive feedback loop,” Jalal explains. “So you’ve grown up being told by your grandmother that spirits and demons inhabit your village after dark. You wake up during REM sleep, you see some kind of a shadow, and you starting panicking, creating more body image hallucinations which your mind interprets in this cultural narrative and so you perceive a demon coming towards you. And then you go to bed the next night even more afraid, so it happens again, and you perhaps start to believe you’re possessed.”

This is thought to be why sleep paralysis is a far more common occurrence in parts of the world where many of these cultural explanations regarding the supernatural still exist.

Once movement returns to the body, the hallucinations disappear almost immediately. The length of the whole episode can last from a few seconds to 20 minutes. Scientists believe that sleep paralysis may be behind many of the medieval folklore narratives describing vampires and ghosts terrorizing villages at night, before suddenly vanishing into the ether.

Such stories stretch back almost to the start of recorded history, but the earliest attempts to seriously define the experience of sleep paralysis, were made by the Greek physician Paulus Aegineta in the 7th century AD.

Aegineta coined the name pan ephialtes, believing it to be the work of Pan, the faun-like God of nature and the wild, leaping upon the chest of his unfortunate victim. Anglo-Saxon England was more preoccupied by the idea of witches descending onto the helpless sleepers trapped in their beds, a concept which entered our lexicon through the word “haggard”, meaning to be “ridden by the hag”.

“It seems to be a major part of culture going back through time,” Sharpless says. “And that’s pretty understandable. I mean, how would you explain it in a pre-scientific world? You go to bed and you wake up and you see a shadowy figure hovering on top of you, doing things to you.”

But while sleep paralysis affects a surprising number of people worldwide, for the majority it remains a freaky, one-off. As a result, there is yet to be a single randomised clinical trial of either medications or psychotherapy for the treatment of sleep paralysis.

For those who experience it as a recurring problem, psychologists have a few simple tips which can help. These include trying to establish a more regular sleep cycle and avoiding sleeping on your back or stomach. “People are statistically less likely to have it, if they sleep on their side,” Sharpless says. “We think there’s something about the extra weight when we’re in a supine position that makes it more likely.”

Jalal is currently working on a novel, direct treatment, but from experience he’s found that simply educating people about the real causes can take away much of the anxiety which is prompting them to persistently waken during REM sleep.

“People don’t tend to make the supernatural attributions anymore but their experiences have such a vivid quality, they tend to think there’s something deeply wrong with them,” Jalal says. “And these days it seems to be more palatable to put it down to extra-terrestrials.”

A 2012 National Geographic survey found that up to 77% of Americans believe there are signs that aliens have visited the Earth and a 2008 poll suggested that 55% are convinced they have had an alien abduction experience.

Perhaps sleep paralysis could be at the root of these findings? “It may well be,” Jalal says. “It’s these cultural explanations which embed themselves into the whole experience. And because it seems so real, it encourages these pockets of beliefs to spread.”