Sleep paralysis is a REM sleep phenomenon. It occurs when REM sleep essentially intrudes into wakefulness. The body is still in REM sleep, so it is paralysed; but the eyes are open and aware of the bedroom. Sometimes that’s the end of it: you’re just semi-awake in your bedroom and can’t move. But sometimes other elements of REM sleep also occur: dreamlike experiences and very heightened emotions, usually intense fear. People who have had sleep paralysis often report a presence in the room, and sometimes see it as well. It may be a ghost, a witch, a demon, or some other malevolent being. Sometimes it even approaches the paralysed dreamer, and may press down on their chest or try to suffocate them. Sleep paralysis is experienced all over the world and the hallucinations that accompany it are remarkably similar cross-culturally, albeit they are interpreted in different culturally appropriate ways. A fantastic short film called Devil in the Room encapsulating this experience has recently been made and can be seen at the Sleep Paralysis Project.