DR MICHAEL MOSLEY: 'Seeing' a ghost is a wake-up call you can mute trends now
Have you ever seen a ghost? I haven't, but my wife, Clare, woke me earlier this week, convinced there were ghosts in our bedroom.
'I can see them moving around at the end of the bed,' she told me. 'They don't have faces and they don't look threatening, but they are definitely there.'
At this point she turned the lights on and insisted that we get out of bed and have a look behind the curtains, and then under the bed. Reassured that the ghosts had gone, she went back to bed and swiftly fell asleep.
In fact, I didn't find this particularly odd because it happens quite frequently. I don't for a moment believe in ghosts, and nor, when she is fully awake, does Clare.
But what she experienced isn't a conventional nightmare, nor is it a sign that she's going crazy. Instead, it's caused by something called sleep paralysis.
But what she experienced isn't a conventional nightmare, nor is it a sign that she's going crazy. Instead, it's caused by something called sleep paralysis
Experiencing sleep paralysis, with or without the ghosts, is surprisingly common.
According to the American Academy of Sleep Medicine, 20 per cent of us experience something like this on a fairly regular basis.
And though most people, like Clare, just think, 'That's weird' and then go back to sleep, many find it genuinely terrifying.
So what is going on? Sleep paralysis occurs during rapid eye movement (REM) sleep, which is when we have our most intense dreams and nightmares.
While we are in REM sleep, our limbs become temporarily paralysed to prevent us thrashing around and hurting ourselves or others; you continue to breathe, and your eyes flicker from side to side, but you can't move.
Most of us are completely unaware that this is happening, because we're asleep, but sometimes your brain wakes you up while you are still in the grip of REM dreams, and that can be disturbing.
Different cultures have different explanations for these intensely vivid experiences.
A 2015 study in Italy, for example, found that 38 per cent of people who reported experiencing sleep paralysis believed a supernatural being called the Pandafeche — a cat-like creature — was to blame.
According to local folklore, the best way to prevent a Pandafeche attack is to have a pile of sand by the bed to throw in its eyes.
Brazilians, on the other hand, are more likely to blame sleep paralysis on a demon called Pisadeira, a witch-like creature with sharp fingernails that walks on people's stomachs when they are sleeping on their backs.
That myth, at least, has something to it because you are much more likely to experience sleep paralysis when you sleep on your back.
You're also more likely to experience it if you're a shift worker or suffer from post-traumatic stress disorder (both of which disrupt sleep) — or if you have a family history of sleep paralysis.
None of this applies in Clare's case. Though it is more likely to happen when she is stressed, and there is some suggestion that sleep paralysis may be stress-related.
As for why people see ghosts, or demons, Professor Baland Jalal, a neuroscientist at Harvard University, has a theory: he suggests that when you wake up and realise that you are paralysed, your brain jumps to the most obvious conclusion — i.e. that there is some supernatural agent involved.
In a state of heightened alertness your brain