There was that time on holiday in Devon, when she was eight, and she screamed so ear-splittingly that someone in the next chalet called the police. Her parents, clad in dressing gowns and dozy from too much sun and wine, had to explain that the unearthly sound had been made by a sleepy little girl and not someone being brutally murdered.

Over the years there were various rituals she had made her long-suffering parents carry out before bed, checking everywhere for bogeymen.

But the bogeymen still somehow snuck in, if not physically, then covering her with their slick shadows until she woke up thrashing in panic.

Sleep paralysis they call it.

Lately, it always follows the same pattern.

First, coming to in the pearly light of her bedroom, the familiar furniture appearing as dark, blocky shapes around her.

Awake.

Then, the creeping figure. There’s a flare of white panic in her mind before the sweet relief of realization comes.

No, wait, it’s just that thing again. It’s just the sleep paralysis.

It’s not real. None of this is real. I’ll wake up soon.

Except … the face doesn’t go away. Instead, it becomes more defined, more corporeal, until it is visible in high-definition detail, hovering above her as she lies there, powerless and unable to move a single muscle.

The faces used to vary. Sometimes it would be an old man or woman with leathery, crinkled skin and cruel, glittery eyes. But lately, it’s always a man, features hidden, eyes staring down at her through slits in a balaclava.

And that’s when she realizes this time is different. She’s not dreaming. This isn’t sleep paralysis. This is happening.

The surge of horror at this realization is always the tipping point. She breaks free from the sleep-state and finds herself shivering, gasping, out of her bed, carpet under her curled toes, back slick with sweat.

But lately, that moment is taking longer to come.

Friends and family have never understood that it’s worse than a ‘bad dream’. That’s OK. She’s used to being a freak. But what makes her feel really lonely – and really scared – is the people who should know, her fellow parasomniacs.

They don’t believe her when she tells them there’s something different about this.

It’s as if he, whoever he is, is somehow … breaking through whatever barrier exists between the waking world and the nightmare one.

But she’s driving herself mad with thoughts like these. That’s not even possible. Is it?

Don’t they say we only use 10 per cent of our brain or something, and the rest is a mystery? It’s only her silly brain playing tricks.

Tonight, she is going to sleep peacefully all the way through. He won’t come this time. She is quite determined.

She lies still and breathes slowly through her nose, eyes closed.

The bedside clock ticks. Radio 4 burbles on …