You wake up in the middle of the night, convinced that an evil figure is lying in wait. You attempt to move, but your body just will not budge. You try to scream, but nothing comes out. The monster draws closer. It may sound like a horror movie scene, but this is the real deal — you’re experiencing sleep paralysis.

This terrifying and mysterious sleep disorder, or parasomnia, has been experienced by people possibly since the dawn of humankind.

And, it may have given rise to numerous ghost stories and mysterious accounts involving “things that go bump in the night.”

This most unsettling experience was first clearly documented in a medical treatise in the 17th century, by Dutch physician Isbrand Van Diembroeck, who wrote about the case of a woman “50 years of age, in good plight [health], strong,” yet who complained of mysterious experiences at night.

“When she was composing her self to sleep,” explains Van Diembroeck, “sometimes she believed the devil lay upon her and held her down, sometimes that she was [choked] by a great dog or thief lying upon her breast, so that she could hardly speak or breathe, and when she endeavored to throw off the [burden], she was not able to stir her members.”

What the woman in Van Dimbroeck’s account likely experienced was a condition that has come to be known as “sleep paralysis.”

Sleep paralysis and hallucinations

The reason why sleep paralysis is so scary is not just because you will suddenly become alert but realize that you are, in fact, unable to move a muscle or utter a sound, but also because this experience is often — as in the case above — accompanied by terrifying hallucinations.
These, as specialized literature has now ascertained, typically fall into three distinct categories:
  1. a sensed presence, or intruder hallucinations, in which the person feels the presence of an evil, threatening individual
  2. incubus hallucinations, in which the person might feel someone or something pressing down uncomfortably, even painfully, on their chest or abdomen, or trying to choke them
  3. vestibular-motor hallucinations, during which the individual thinks that they are floating, flying, or moving — these may also sometimes include out-of-body experiences, in which a person thinks that their spirit or mind has left their body and is moving and observing events from above
Among the types of dreamlike hallucinations listed above, the first type — a sensed presence — is one of the most commonly experienced by people with sleep paralysis.
As for the time of sleep at which sleep paralysis — with or without hallucinations — normally takes place, again, there is no single answer.
According to a study that was published in the Journal of Sleep Research, sleep paralysis typically happens soon after falling asleep (or hypnagogic episodes), at some point during the course of sleep (or hypnomesic episodes), or a little before the person’s usual time of awakening (or hypnopompic episodes).
The authors of that study note that the most common instances of sleep paralysis are hypnomesic, and that they usually take place after 1–3 hours from falling asleep.


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