In the middle of the night a few months ago I woke up with the terrifying sensation of being crushed in my bed - not by any living thing but by a distinctly evil presence. I know it sounds crazy, but the experience left me quaking and I spent the rest of the night sitting up in bed with the lights on.

In my search to find the source of my "night fright" I stumbled upon the website of Al Cheyne, the head of the department of psychology at the University of Waterloo in Ontario, Canada.

The first thing he did was to put a name to the experience; sleep paralysis. And I was relieved to find out that it's a relatively common sleep disorder. Professor Cheyne:

"Sleep paralysis is in the first instance a brief period of complete paralysis when you are either waking or falling asleep. I suspect it comes down to a fairly minor anomaly in the brain stem that regulates waking and sleeping and that for some of us there's a loose switch of an electrochemical sort which is out of balance."

Evil presence
The result is that your brain is awake, but you can't move. "A fairly large proportion of people will have a number of other experiences," Professor Cheyne says. "They may feel 'a presence' in the room watching them. They may see and hear things approaching them or attacking them. It can be terrifying, in fact many people describe it as the most terrifying event in their lives."

Click to hear programme in Realplayer

That's true for sleep paralysis-sufferer Andrea Parkes:
"I could see this thing high up in the corner of the room. I tried to remain very calm and this thing got annoyed that I was staying so calm, so without touching me it slammed me against the walls, up through the ceiling and down through the floors and then the terror overtook me."

 "The terror comes from losing control," says Hal Crawford, who experiences sleep paralysis several times a year.

"It comes from being in a situation where you don't know what's going to happen next, you don't know if your breathing is going to stop or whether something otherworldly is there. In a way you're battling against yourself and you don't have the power to control your body."

Folk tales
The phenomenon of sleep paralysis is so widespread it has become part of folklore around the world. "Many traditional societies include references to sleep paralysis as part of their everyday sets of working knowledge," Professor Cheyne says. It has survived in Newfoundland, an island off the east coast of Canada.

"Particularly in the small outpost communities everyone knows what the 'old hag' experience is. So someone might get up in the morning and say 'oh I was hagged last night' and everyone knows exactly what happened to them. People in these traditional societies have a great range of explanations for the experience - everything from a condition of the blood, to vampires, to being haunted by an old woman . . . this knowledge seems to have disappeared from urban societies."

In fact in urban societies you're more likely to have people describe their experience as one of 'alien abduction', according to Professor Cheyne. "The little grey men have become a modern icon, so it's not surprising that they are stored somewhere in our memory." And our brain grasps these images to explain the 'fear' it senses.

"The emotion of fear accompanies threats and dangers. When you activate fear you activate a whole set of strategies to deal with danger. If you consider the fact that the part of your brain responsible for fear [the limbic system] is active at the same time that you're awake and paralysed and helpless, this would tend to aggravate this condition."

Essentially your brain strains to find clues to understand what it perceives as a threat. The sound of the fan becomes a whispering voice, a creak becomes a demon climbing on the bed, and when the brain doesn't have a stimulus, it makes one up.

Despite this rational explanation, I still shudder at the thought of the evil presence I sensed in my room. It was so real I find it hard to believe that my brain could fabricate such a terrifying hoax. I'm enormously relieved that in the cold light of day I can refer to Professor Cheyne's scientific understanding of this 'otherworldly' phenomenon, even if this leads in turn to the disturbing realization that our own minds are potentially the source of midnight's worst horrors.

Sharon Shag had The Hag

Almost every night
She'd lie awake for hours
Paralyzed with fright

"There's no such thing as an Old Hag!"
Sharon's mother said.
"That's just some silly notion
You've got inside your head."

From that time on, she slept quite well
Each night passed like the other.
She slept in peace, because The Hag
Was haunting Sharon's mother.