A sleep paralysis demon, witch, ghost, beast, or other ominous creature is a hypnopompic hallucination, sometimes specifically called the incubus phenomenon. The demons often take the shape of childhood fairytale villains. However, more research is needed to understand exactly what is happening in the brain during these demonic hallucinations.

Sleep demons are frightening phenomena that are all too real for people who experience sleep paralysis. Roughly 20% of people have an episode of sleep paralysis at least occasionally. In as many as 75% of these episodes, the sleeper has a hallucination in which they hear, see, feel, or sense something in their bedroom.