Bad dreams in Roman times were attributed to demons called the incubus and succubus. The incubus (from the Latin incubare, to lie on, which also gives us the word incubate) was said to consort with women in their sleep. This explained the heaviness or suffocation a woman might feel during a bad dream. The superstition was so widely believed in the Middle Ages that any woman who gave birth to a witch was supposed to have been visited by a male demon. The female counterpart of the incubus was the succubus (from the Latin suc or sub, under, and cubare, to lie), who slid beneath a male sleeper and gave him a terrible ride. There were laws against these demons in the Dark Ages, and their existence was recognized by the Church and the state. Soon they became known as the night hag, the riding of the witches, and the nightmare as well. The nightmare, however, had nothing to do with a female horse, taking its name from night, plus the Anglo-Saxon mare, meaning incubus. Nightmare eventually replaced the older Latin word incubus, which is now used to describe an oppressive load. Although the word wasnt born from a horse, it caught the popular imagination because of the graphic picture it suggested of a terrible horse bearing sleepers off on a frightening ride.