There are countless stories of people waking up in the night after they have gone to bed, and they report a menacing presence in the room or even on top of them that they can feel and see.

Those who have experienced these episodes talk of extreme fear, being unable to scream and having a feeling that their body is paralyzed or are being held down by something or someone. Others speak of the entity in the room squeezing their throat or their chest, meaning they feel like they are slowly suffocating.

For some, it’s a faceless, shapeless presence trying to choke them. Others describe a creepy old hag, a serial killer with a mask or just faceless, or a full alien abduction. For some, the demons look like dead relatives.

It has also been called the incubus phenomenon: an "attack" by a male demon on a woman. (Its female counterpart, the succubus, usually attacks men.) For centuries, the incubus demon has been said to haunt sleepers, inspiring tales in traditional folklore and works of art.

Dr Jan Dirk Blom, a professor of clinical psychopathology at the University of Leiden in the Netherlands, describes the feeling as, "Lying in bed in such a state of paralysis, the brain's threat-activated vigilance system kicks in and helps to create a compound hallucination of a creature sitting on the chest.”

What the afflicted person sees is a combination of their actual surroundings and a nightmare, which is projected onto the real world. The experience feels exceptionally real.

What causes these strange sleep episodes?

Sleep Paralysis

The phenomenon called “sleep paralysis” is a state during waking up or falling asleep in which a person is aware but unable to move or speak. During an episode, you may hallucinate (hear, feel, or see things that are not there), and a feeling of fear accompanies it.

It happens when you wake up during the dream phase of sleep, and the underlying mechanism is believed to involve a dysfunction in REM (Rapid Eye Movement) sleep. During REM sleep, which is the period when a person typically dreams, the body's muscles are relaxed to the level of paralysis to prevent the sleeper from acting out his or her dreams and injuring him or herself.

But when sleep paralysis occurs, the person's mind wakes up, but the person is still dreaming, and the body is still paralyzed. As a result, if you suddenly wake up while still in this dream phase, you’re fully conscious but unable to move as your body’s muscles are inactive.

Between 8% and 50% of people experience sleep paralysis at some point in their life, and about 5% of people have regular episodes. Males and females are affected equally.

Canadian Inuits say that the spells of shamans cause sleep paralysis. African and Japanese folklore says it’s a vengeful spirit that suffocates its enemies in their sleep.

In Brazil, a demon called Pisadeira, which is Portuguese for “she who steps.” is blamed. She is a creature with long fingernails who walks on the chest of people who sleep on their backs.

Hypnagogic or hypnopompic hallucination

Hypnagogic or hypnopompic hallucination describes the experience of seeing entities around or on the bed, and they can often be mistaken for nightmares. These episodes can seem real and are often frightening. They occur when someone is falling asleep (hypnagogic) or waking up (hypnopompic).

These hallucinations are often accompanied by sleep paralysis and can happen if you’re partially conscious during the rapid eye movement (REM) cycle of sleep.

You might also see a distortion of something that is there. For instance, the pile of clothes on your chair could turn into someone sitting there watching you sleep.

These episodes can be increased in frequency by anxiety, lack of sleep, jet lag, and excess alcohol consumption.

Rodney Ascher is a film director and sleep paralysis sufferer who released The Nightmare in 2015, which explored the first-hand experiences of people who suffer from sleep paralysis. “I interviewed people who believe that their experience happened in a state of consciousness where they’re more sensitive to the supernatural,” he says. “Most of them reported seeing variations of shadow people, including something close to the Grim Reaper, but other intruders appeared as ghosts, mechanical claws, cats and even something similar to aliens.”

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