But this sleep system isn’t foolproof. A combination of neurotransmitters is believed to be responsible for paralysis during REM sleep, acting together to switch off motoneuron activity. During normal sleep, the ability to move is switched back on before a sleeper wakes up. But for reasons that aren’t fully understood, sometimes things malfunction and the sleeper becomes conscious but remains frozen. When this occurs, the amygdala, the part of the brain that detects threats in the environment, triggers a primal fight-or-flight reaction. The brain screams to flee, but the body doesn’t react. The stuck person finds themselves in a blended state of consciousness: awake and aware of their surroundings, yet near enough to sleep to still experience dream-like hallucinations. The terror induced by being awake yet paralyzed most often drives the hallucinations towards the dark and disturbing.

According to James Cheyne, a psychologist at the University of Waterloo, up to 40 percent of people experience sleep paralysis once or twice in their lives. Usually they dismiss it as a terrifying—albeit extremely unusual—dream. Up to 6 percent of people (myself included) report multiple episodes over extended periods. In recent years, research by scholars such as Cheyne, who is one the world’s leading experts in sleep paralysis, and a flurry of media attention have increased public awareness of this bizarre state. But the scientific work also casts new light on a range of cultural beliefs across the globe. From ancient tales of demonic night visitors to contemporary reports of alien abductions, there’s a long history of terrifying tales that may be linked to sleep paralysis. These stories support a notion that while long-mysterious, the phenomenon is as common as sleep walking.

Prior to neurobiological explanations, which only have become possible in the past few decades, people had to rely on folklore to make sense of these nocturnal terrors. The accounts stretch back to antiquity, with remarkable consistency. Descriptions of sleep paralysis world-over share a sense of an intruder or presence in the room; painful feelings of being crushed, dragged, or touched; and visual, tactile, and aural hallucinations that range from laughing devils and demonic dogs to black shadowy figures and sex-crazed witches.

These descriptions have been especially bountiful in literature and art. In Herman Melville’s 1851 novel Moby Dick, for example, Ishmael, the narrator, awakens to find himself unable to move and horrified to find that a “supernatural hand” appeared to be placed in his own:

My arm hung over the counterpane, and the nameless, unimaginable silent form or phantom, to which the hand belonged, seemed closely seated by my bedside. For what seemed ages piled on ages, I lay there, frozen with the most awful fears, not daring to drag away my hand; yet ever thinking that if I could but stir it one single inch, the horrid spell would be broken.

Another classic portrayal—and the illustration accompanies almost every article on sleep paralysis—is Henri Fuseli’s The Nightmare, a 1781 painting that depicts a woman lying on her back with a demon on her chest and a ghoulish mare lurking background. Pressure on the chest and the sense of being watched are common characteristics of sleep paralysis. Psychiatrist Jerome M. Schneck connected sleep paralysis to The Nightmare in 1969, and it is an interpretation that continues to be cited across research to this day.