Have you ever woken up in the middle of the night and found that you are unable to move, or perhaps even worse, seen shadowy figures in your room that terrified you as much as not being able to move?
"I woke in the middle of the night but when I tried to move my hands and legs, I couldn't," Kirsty Corbett, 32, told PA.
"It was terrifying. I couldn't even scream out for help. I thought I was dead, but then suddenly, I was able to move again. I was really confused, and just put it down to a bad dream. After that, it happened again and again."
If like Corbett, you're sure you haven't been drugged by intruders or visited by aliens and ghosts, there is a scientific explanation – sleep paralysis.
It is currently thought that only about 6% of people in the UK suffer from the condition, which can last for anything from a few seconds to several minutes.
However, as it is very difficult to test for it, there is very little knowledge about how or why it occurs and there could be a higher number of people affected.
A team of researchers from University of California, San Diego have come up with a theory about why sleep paralysis occurs.
Their paper, entitled Sleep paralysis and 'the bedroom intruder': The role of the right superior parietal, phantom pain and body image projection is published in the journal Medical Hypotheses.
Waking up during REM sleep
From conducting interviews with people who suffer from the condition, the researchers think that sleep paralysis occurs when a person wakes up during the rapid eye movement (REM) stage of sleep.
The feeling of not being able to move one's limbs and muscles for a while could potentially be an evolutionary adaptation of the brain, designed to keep people from acting out their dreams, and it takes a while for the brain to realise that the person is awake.
As for the hallucinations of shadowy figures, the researchers postulate that this could be caused by a disturbance in the part of the brain that holds a hardwired neural map of the body or the "self".
"Perhaps, in part of the brain, there's a genetically hardwired image of the body — a template," Baland Jalal, a neuroscientist at the University of California, San Diego told Live Science.
The researchers theorise that this hardwired image could be located in the parietal lobes, which are situated in the top-middle part of the brain.
When a person suddenly wakes up during REM sleep, the parietal lobes monitor neurons in the brain that are firing commands for the muscles move, but due to the evolutionary aspect, the limbs are temporarily paralysed, causing a disturbance.
Fear can potentially prolong episodes
Due to the disturbance, the brain builds up an incorrect sense of its neural image of the body, which could manifest itself as a hallucination.
Jalal, who has been studying sleep paralysis for several years, previously found a connection between the length of the episodes and how people thought about them in a 2013 study conducted with Harvard Medical School.
Analysing people in Egypt, who are mostly very religious, against people suffering from the same condition in Denmark, who are largely atheist, Jalal found that Egyptians believed that the hallucination and sleep paralysis was inflicted by a jinn (a ghostly demon-like creature) from Islamic mythology.
The Egyptians experienced a great deal of fear due to these beliefs and were more likely to experience prolonged episodes, compared to the Danish participants, who felt that the sleep paralysis was caused by physiological factors such as sleeping the wrong way or some sort of brain malfunction.
"If you have fear, the activation in fear centres in the brain might mean more likelihood of fully awaking during sleep paralysis, and experiencing the whole thing," Jalal said. "And by experiencing it, you would have more fear — and then, you have all these cultural ideas of what it is added as well, and now you are even more scared of it."