The painting called "the nightmare" is one of the classic depictions of sleep paralysis, when people become conscious after sleep but are unable to move because their muscles are so relaxed. For some people it can be terrifying.
"There are certain things which come up again and again including a sense -- a very strong sense -- there's a presence in the room with you, there's something, usually something evil," says Carla MacKinnon. She has directed a documentary film about sleep paralysis for the Royal College of Arts in London. Her own experiences with sleep paralysis led her to work on the documentary.
"I wanted to find out why people seem to get these same kind of narratives that come up again and again," she says, "I started looking into the mythology of it."
There have been myths and folklore: from witches casting spells over people to demons haunting their bedrooms. But there's a more realistic explanation about what causes sleep paralysis.
Dr. Steven Feinsilver, director of the center for sleep medicine at Mt. Sinai School of Medicine, says anxiety and stress are often to blame. He says it is normal for your muscles to be relaxed during dreams.
"The problem is if you are awake during that time... you can have the sensation that you can't move, which can be really frightening," Feinsilver says. He estimates that at least one in 10 people has experienced sleep paralysis and that it is actually more common than people even realize.
"If I were to ask the general public, 'Have you ever had the feeling as you were waking up, particularly, that you were kind of awake but you couldn't move?' I think a lot of people might agree with that," he says, adding that the best treatment is reassuring people that they are not going crazy.
"It's not going to hurt you," he says, "it can be really annoying."
So, don't worry about demons climbing under the covers with you or evil spirits haunting you.
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