I was a teenager when it first happened. It was the early hours of the morning, still some hours before I had to get out of bed for school. I woke up and tried to turn over in bed, but my body wouldn't let me – I was unable to move, paralysed down to my toes.
Although my brain was conscious, my muscles were still asleep. My bedroom felt hot and restrictive, like the walls were closing in and I felt panicked. Finally, after about 15 seconds, the paralysis lifted.
Later, I found a name for what had happened to me: sleep paralysis. It's a surprisingly common night-time condition in which part of your brain wakes up while your body remains temporarily paralysed. After that initial scary incident, it became a frequent occurrence, with an episode every two or three nights. The more it happened, the less frightening it became. Eventually it was little more than an inconvenience.
But sleep paralysis can be far more life-affecting. And for some, it comes with terrifying hallucinations. One 24-year-old sufferer I spoke to, who asked to be only identified by her first name, Victoria, remembers it happening one night when she was 18. "I woke up and couldn't move," she says. "I saw this gremlin-looking figure hiding behind my curtain. It jumped on my chest. I thought I'd entered another dimension. And the scariest thing was I couldn't scream. It was so vivid, so real."
Others hallucinate demons, ghosts, aliens, threatening intruders, even dead relatives. They see parts of their own bodies floating in the air, or cloned copies of themselves standing beside their bed. Some see angels and later believe they have had a religious experience. Researchers think these hallucinations may have fuelled belief in witches in Early Modern Europe, and could even explain some modern claims of alien abductions.
Scientists think sleep paralysis has probably existed for as long as humans have slept. There are several colourful descriptions of the episodes through literary history, and Mary Shelley was apparently inspired to write a scene in Frankenstein by a painting depicting an episode of sleep paralysis. But until recently it has been little researched. "It's been an ignored phenomenon … but over the last 10 years there's been a growing interest," says Baland Jalal, a sleep researcher at Harvard University who in 2020 completed what may have been the first clinical trial into different ways of treating sleep paralysis.
Jalal is one of a handful sleep scientists now investing serious time and energy into researching the condition. They hope to paint a more robust picture of the causes and effects – and find out what the condition tells us about the broader mysteries of the human brain.
Until recently, there was little agreement about how many people experience sleep paralysis. Studies were sporadic, with little consistency between methods.
But in 2011, clinical psychologist Brian Sharpless, currently a visiting associate professor at St Mary's College of Maryland, conducted the most comprehensive review to date of the condition's prevalence while he was at Pennsylvania State University. It looked at data from 35 studies spanning five decades. Collectively they included more than 36,000 volunteers. Sharpless found that sleep paralysis was more common than previously thought, with almost 8% of adults claiming to have experienced it at one point. That figure is much higher among university students (28%) and psychiatric patients (32%).
"It's really not that uncommon," says Sharpless, who is also the co-author of Sleep Paralysis: Historical, Psychological, and Medical Perspectives.
After experiencing the condition, some gravitate towards supernatural or even paranormal explanations. In reality, says Jalal, the cause is far more mundane. At night, our body cycles through four stages of sleep. The final stage is called rapid-eye movement sleep, or "REM". This is when we dream. During REM, your brain paralyses your muscles, probably to stop you physically acting out your dreams and hurting yourself. But sometimes – and scientists still aren't sure why – the sensory part of your brain emerges from REM prematurely. This makes you feel awake. But the lower part of your brain is still in REM, says Jalal, and is still sending out neurotransmitters to paralyse your muscles.
"The sensory part of the brain becomes active," says Jalal. "You're waking up mentally, perceptually – but physically you're still paralysed."
Back in my early twenties I experienced sleep paralysis every two or three nights, but even then, it didn't have much impact on my life. It was an interesting anecdote for friends and family. In that respect, my experience was common. "For the majority of people, it's a quirky thing they live with," says Colin Espie, professor of sleep medicine at the University of Oxford. "It's a bit like sleepwalking – most people who sleepwalk don't ever see a doctor. It's a curiosity in the family, a talking point."
But for an unlucky minority, the condition proves more testing. Sharpless's research found that between 15% and 44% of those with sleep paralysis experience "clinically significant distress" as a result. The problems usually arise from how we respond to sleep paralysis, rather than the condition itself. Patients find themselves obsessing throughout the daytime about when the next episode might strike.
"It can lead to anxiety at the beginning and end of the night," says Espie. "You grow a network of worry and concern around it. The worst expression of that is turning into a kind of panic attack."
In the most serious cases, sleep paralysis may be a sign of underlying narcolepsy – a more serious sleep condition in which the brain is unable to regulate sleeping and waking patterns, causing someone to fall asleep at inappropriate times.
Doctors say paralysis is more likely to occur when you are sleep deprived, because your sleep architecture is fragmented. Some sufferers also find it is more likely when lying on their back, though the explanation for this is unclear.
The most common approach to treating sleep paralysis is educational: patients are simply taught about the science behind the condition, and reassured that they are in no danger. Sometimes a form of meditation therapy will be used. The aim is to reduce the patient's anxiety around going to bed, and to train them to remain calm when sleep paralysis strikes.
In more serious cases, drugs may be considered – including selective serotonin reuptake inhibitors (SSRIs), which are normally used to treat depression but have a side-effect of suppressing REM sleep.
The most dramatic and memorable episodes of sleep paralysis are normally those that come with vivid hallucinations. Usually these nocturnal visions are a source of fear, but scientists also think they can tell us fascinating things about the human brain.
When you enter sleep paralysis, your brain's motor cortex starts sending signals to the body, telling it to move. But the muscles are paralysed, and so the brain does not get any feedback signals in return. "There's an incongruence … the self is broken up, degraded," says Jalal. As a result, the brain "fills in the gap", and creates its own explanation for why the muscles cannot move. That's why so many hallucinations involve a creature sitting on your chest or holding your body down.
It reinforces the idea, popular among evolutionary scientists, of the human brain as a "storytelling machine". We struggle to accept the fact that much of the world is random, and so our brain devises dramatic narratives in an effort to find meaning in the mundane.
Christopher French, head of the anomalistic psychology research unit at Goldsmiths, University of London, has spent more than a decade speaking to people across the world who have experienced these hallucinations, and recording what they saw. "There are common themes, but there's also a huge amount of idiosyncrasy, variability," French says.
Some hallucinations are difficult to explain – and even downright bizarre. Over the years French has recorded sightings of a sinister-looking black cat, and a man being strangled by plants. But others are far more common and seem to be heavily influenced by culture. In Canada's Newfoundland, it's common to see an "Old Hag" sitting on your chest. Mexicans report a "dead man" lying on their chest, whilst St Lucians speak of "kokma", the souls of unbaptised children, strangling them in their sleep. Turks describe the "Karabasan" – a mysterious, ghostly creature. Italians often hallucinate witches.
It strengthens the idea of humans as overwhelmingly social animals, influenced heavily by culture and expectations.
Indeed, in a series of studies Jalal compared symptoms in Denmark and Egypt, among volunteers with a similar age and gender distribution, and found a cultural chasm in how sleep paralysis manifested. Egyptians were far more likely than Danes to have experienced sleep paralysis (44% compared to 25%), and were more likely to endorse a supernatural explanation. The Egyptian volunteers who believed in ghosts and demons also spent a longer time paralysed during each episode.
Jalal's theory is that fear of the supernatural makes people more afraid of sleep paralysis, and this anxiety makes the phenomenon more likely to occur – a demonstration of the close fusion between our minds and bodies.
"When you have anxiety and stress, your sleep architecture will become more fragmented, so you're more likely to have sleep paralysis," he says. "Let's say your grandmother tells you, 'The creature looks like this, it comes at night and attacks you'. And because of this fear, [you're] hyper aroused, the fear centres of your brain are hyper alert. And lo and behold, during REM sleep [you] feel, 'Oh, something is wrong, I can't move, the creature's here'.
"It seems like culture can really create this striking effect."