An evil spirit's name became a word for a bad dream—which?

The Mare in the Dark

In Anglo-Saxon England, the 'mare' was no horse. It was a demon, specifically female, who crept through keyholes and crouched on the chests of sleepers, pinning them in a breathless, paralyzed state between waking and sleep. The Old English 'mare' shares roots with the Proto-Germanic '*marōn,' meaning 'crusher' or 'oppressor,' and by the year 1000 the compound 'niht-mare'—night-crusher—was common currency in medical texts and folklore. The Scots called it the 'mair,' the Dutch the 'nachtmerrie,' the Icelandic the 'mara.' Descriptions were startlingly consistent: victims woke unable to move or cry out, sensing a malevolent presence in the room, often seeing a shadowy figure or feeling crushing weight on the sternum. In 1639, the physician John Browne wrote of patients 'tormented by the Mare' and prescribed everything from iron horseshoes hung over beds to prayers muttered backward. The nightmare was not a vision you had—it was a creature that had you.

When the Demon Became the Dream

For centuries, 'nightmare' meant only the demon. If you wanted to describe a frightening dream, you said 'a fearful dream' or 'a vision of dread.' Shakespeare used 'dream' and 'vision' for nocturnal horrors; 'nightmare' appears nowhere in his works. The semantic shift began in the 1700s, when Enlightenment physicians started attributing the phenomenon to indigestion, bad air, or nervous disorders rather than demonic visitation. By 1768, the painter Henry Fuseli could title his famous canvas 'The Nightmare' and depict both the incubus squatting on a sleeping woman's chest and the wild-eyed horse emerging from curtains—visual puns on both the old and new meanings. Samuel Johnson's 1755 dictionary defined 'nightmare' as 'a morbid oppression in the night, resembling the pressure of weight upon the breast,' still clinging to the physical sensation. But by 1828, Noah Webster's American dictionary included 'a terrifying dream' as the primary definition, and the demon had been fully linguistically domesticated into a metaphor. The creature became the experience, and the word we inherited names a ghost that no longer haunts.

The Biology Behind the Belief

What medieval Englishmen called the mare, modern neuroscience calls sleep paralysis—a glitch in the brain's transition between REM sleep and wakefulness. During REM, the body enters atonia, a temporary paralysis that prevents us from acting out dreams. Normally this shuts off the instant you wake. In sleep paralysis, the atonia lingers while consciousness returns, leaving the victim awake, aware, and utterly immobile. The brainstem's pons and medulla are still issuing 'freeze' signals to the limbs. Meanwhile, the amygdala—the brain's fear center—remains hyperactive from the dream state, flooding the conscious mind with dread in the absence of any external threat. In a 2011 study at the University of Waterloo, 8 percent of the general population reported at least one episode, with incidence spiking among shift workers, narcoleptics, and the sleep-deprived. Hallucinations are common: the brain, receiving no sensory input from the paralyzed body, invents explanations for the terror and immobility, conjuring shadowy intruders, crushing weights, or the sensation of being dragged from bed. Cultures worldwide have named the phenomenon—Newfoundland's 'Old Hag,' Japan's 'kanashibari,' Mexico's 'se me subió el muerto'—but all describe the same neurological event our ancestors personified as the mare.

Why the Word Still Haunts Us

The nightmare survives in English not because we believe in demons, but because the metaphor is too vivid to abandon. We say 'living nightmare' for waking horror, 'nightmare scenario' for catastrophic outcomes, 'bureaucratic nightmare' for Kafkaesque frustration—the word has become our go-to intensifier for dread. In 1992, the FDA even coined 'drug-induced nightmare' as a formal adverse-event category, codifying a folk term into medical taxonomy. Meanwhile, sleep paralysis remains underdiagnosed and often mistaken for psychiatric disorder; a 2017 review in *Sleep Medicine Reviews* found that fewer than half of sufferers ever mention it to a doctor, and many still interpret the experience through cultural or supernatural lenses. The mare may be dead, but her linguistic ghost presses on our chests every time we describe a bad dream, a rough day, or a policy disaster—and every Halloween, when costume shops sell 'nightmare' masks, they resurrect the demon one more time, proof that a word can outlive the creature it named and still carry its weight a thousand years on.