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Nightmares are scary and unpleasant. But you can rest easy knowing that the fascinating origin of the word nightmare makes it clear humans have been having them for hundreds of years.

In Old English, a mare was a kind of evil or cursed spirit. Mares appear in all kinds of folklore, including German and Slavic stories. Mares were said to ride on people’s chests at night, causing suffocation and bad dreams. These mares, often female, were known as nightmares (because they came at night).

By the 16th century, the word nightmare came to refer to a sensation of suffocation or anxiety during sleep, and now simply a bad dream. While nightmares are terrifying, there is good news: at least most of us don’t worry about evil spirits trying to suffocate us in our sleep anymore.

The roots of these other words may get a rise—of laughter or surprise—out of you. Run on over to our roundup of them at “Weird �y��vlog Origins That Will Make Your Family Laugh.”

Nightmares can have many causes—but evil spirits aren’t one of them, despite what Freddy Krueger might say. Stress, eating before bed, medication side effects, and sleep disorders can all cause bad dreams. While children are more likely to have them, half of all adults also report regularly having nightmares.

In fact, nightmares are so familiar (and frightening) to people that the word nightmare has been metaphorically extended to any terrible thought, experience, or situation that resembles a nightmare (e.g., Being lost at sea was a living nightmare).

Worth noting: A night terror is a sudden feeling of extreme fear that awakens a sleeping person, usually during slow-wave sleep, but it is not associated with a dream or nightmare.

The -mare in nightmare doesn’t have anything to do with a mare as in an adult female horse. This homophony hasn’t stopped the card game Magic: The Gathering and other works of pop culture from concocting fictional demonic horse characters that terrify people at night and which go by punny names like Nightmare.