Lying in her bed in the middle of the night, Elizabeth Earle woke suddenly to see a menacing shadow in her room. She tried to scream, but couldn't open her mouth.

What Ms Earle felt was 'sleep paralysis' – a terrifying phenomenon that 40 per cent of people experience at some point in their lives.

Now US researchers believe they know why this strange experience occurs, and it's all to do with a mix-up in an area of the brain that holds a neural map of the 'self'.

In sleep paralysis, the person wakes up but the transition between sleep and waking up isn’t as smooth as it should be. They begin to wake and may even be able to open their eyes, but find themselves unable to move

They believe sleep paralysis takes place when a person wakes up during a stage of sleep known as rapid eye movement (REM), in which they are usually dreaming.

REM lasts for around five to 15 minutes and is repeated roughly every 90 minutes throughout the night.

Although the person is awake, their muscles are nearly paralysed, which may be an evolutionary device to keep people from sleep walking while dreaming.

Scientists at University of California, San Diego, say that one explanation is that there's a disturbance in the brain region that holds a neural map of the 'self.' The researchers suggest that this hardwired image could be located in the parietal lobes, the top-middle part of the brain (in yellow)

WHAT IS SLEEP PARALYSIS? Sleep paralysis occurs during rapid eye movement (REM) sleep or dream sleep. REM lasts for around five to 15 minutes and is repeated roughly every 90 minutes throughout the night. During this time, the brain turns off most of the body’s muscle function, temporarily paralysing muscles. In sleep paralysis, the person wakes up but the transition between sleep and waking up fully isn’t as smooth as it should be. They begin to wake up and may sometimes even be able to partially open their eyes, but find themselves unable to move. The paralysis only lasts for a few seconds or minutes but can be terrifying because the victim may experience vivid hallucinations. Adrian Williams, professor of sleep medicine and consultant at the London Sleep Centre and Guys and St Thomas’ Hospital, said: ‘As the person has partially woken up the hallucinations can feel very real. 'They often involve hearing, or seeing ghosts or other menacing figures. Some sufferers even feel they are being touched, sat on, or pulled off the bed.’ Advertisement

But among those who experience sleep paralysis, a small group of people also feel as though there is a demonic figure in the room, pressing down on their chest.

Scientists at University of California, San Diego, say that one explanation is that there's a disturbance in the brain region that holds a neural map of 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 suggest that this hardwired image could be located in the parietal lobes - 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 command muscles to move.

However, the limbs are temporarily paralysed, causing a disturbance which manifests itself as a hallucination.

The appearance of a ghost or demon could result when the brain tries to project the person's own body image onto a hallucinated figure, said Dr Jalal.

If this idea is true, people who are missing a limb might hallucinate figures who are missing the same limb, he added said.

The 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.