It sounds like a scene out of a horror movie: You wake up in the middle of the night, out of a vivid dream, to find yourself unable to move, your arms and legs don't respond to commands from your brain, you can't even cry out for help.

It's not a scenario dreamed up by Freddy Krueger, but rather a mysterious ailment that affects nearly 8 percent of the world's population, according to The Guardian. Its name? Sleep paralysis. 

According to Live Science, sleep paralysis occurs when a person wakes up while their brain is still in rapid eye movement (REM) sleep. During this period of time, the brain "turns off" the muscles of a body so that a person does not perform the movements of their dreams and or suffer injuries. 

Even worse, because the brain is still in REM mode, it can project very realistic, hallucinatory images for the person undergoing the paralysis, according to the Guardian.

"I had one patient who was lying in bed and woke up to see a little vampire girl with blood coming out of her mouth," Washington State University clinical psychologist Brian Sharpless told the Guardian. "This is an example of a really vivid, multi-sensory hallucination. She could feel this vampire figure grabbing onto her arms, pulling her, and saying she was going to drag her to hell and do all these terrible things to her."