Have you ever suffered an episode of sleep paralysis? It can be a tremendously frightening experience, one you're not likely to forget. You wake from sleep immobilized, unable to move your body or turn your head. You try to make a sound, but you can't open your mouth. You feel short of breath, with a strong feeling of pressure on your chest, weighing your body down. You feel a deep sense of dread or danger -- maybe you even feel there's a strange presence in the room.