Novelist Bella Pollen had always been a bad sleeper. She was regularly awake at 4am, too early, she says, to start the day and too late to take a sleeping pill.
And it was 4am precisely that she heard the tread of unfamiliar footsteps coming up the stairs.
'I was alone in the house which was still a building site. My bed was pretty much the only piece of furniture in there. I remembered thinking, somewhat irrationally, that there couldn't be an intruder because I'd locked all the doors and windows.
'Yet something was climbing those stairs. The door handle moved and almost immediately I felt the bed dip and a pair of arms encircle me from behind.
'On some level, I knew I must be dreaming, that I was below the level of conscious reasoning, because instead of being terrified I found it strangely comforting. I felt safe, cherished even.'
Swiftly she realised her error. 'Every time I breathed out, the arms seemed to squeeze me tighter and tighter to the point where I could not get enough air into my lungs.
'I began to panic. I tried screaming but no sound came out. I tried fighting but my limbs were paralysed. Eventually, I mustered enough strength to break free.'
Author Bella Pollen (pictured) has written about the effects of her unusual sleep paralysis in her memoir Meet Me In The In-Between
Bella leapt violently out of bed and went to the bathroom to wash her face, believing it had been nothing more than a particularly vivid nightmare.
She slid back under the duvet, reassuring herself that she was now awake. 'I could see the minutes ticking past on my bedside clock. How could I not be?' she asks.
What happened next would be provocative enough were it a chapter in one of her bestselling books.
Bella, 56, a mother-of-four, and married to David Macmillan of the Macmillan publishing empire, felt the entire scene repeat itself.
'The bed dipped, the same arms gathered me in. It was a male figure, featureless but very strong.
'The pressure built. I began to choke, but suddenly, unexpectedly, rippling up through the terror, I felt a savage, almost primordial arousal.'
The following night over dinner the novelist told her husband and friends about her visitation.
Her guest, the late writer A. A. Gill, immediately identified it as a paranormal phenomenon called an incubus. 'I looked it up,' remembers Bella.
'It said an incubus was a demon who lies upon sleepers, especially women, in order to engage in sexual activity with them, and that it was a phenomenon recorded in every corner of the globe.
'In Ecuador there's a dwarf who particularly likes hairy women and serenades them with a guitar.
'In Brazil there's a dolphin disguised as a handsome man who drags women to the river with a hat covering his blowhole. German folklore describes a winged goblin.
'I had a man who I can best describe as being made out of iron filings magnetically held together. By comparison, it was not so weird after all.'
Behind all the demonic folklore lies a more earthly medical issue.
Folklore: An 1800 painting by Nicolai Abraham Abildgaard depicting an incubus
As Bella was to discover, she was in fact suffering from a classic case of sleep paralysis, a neurological disorder where rapid eye movement (REM) sleep continues after a person has woken up.
In a REM cycle, the brain has vivid dreams but the body is paralysed to prevent people trying to act them out.
'Only the eyes and the muscles needed to breathe are fully functional.
When this state extends into wakefulness, hallucinations, such as Bella's 'demon' are a common response, as is an overwhelming fear.
A sense of sexual arousal – and satisfaction – is also a normal symptom.
Neurologists cannot explain sleep paralysis, other than to say it is part of the brain and body's confusion at being simultaneously awake and asleep.
For Bella, knowing what it was did not prevent it happening again.
In the days which followed, the demon returned twice more.
Bella has written candidly about the encounters in her acclaimed memoir Meet Me In The In-Between.
'Each time,' she writes, 'the presence returned a few minutes after I had initially broken free, to push into the empty crevices of my body, take me to the edge of the sexual abyss and then carelessly drop me over, adding a frisson of shame to what was already a profoundly frightening experience.
'I was being defiled, brutalised. The creature had worked its full will upon every inch of my body, and yet, yet… I was taking pleasure in it?'
She was as baffled as anyone by the visitations from what she now archly