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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."
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.
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
Wow lucky my brain not so violent to me!!! 
