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.