Sleep paralysis is a condition where you wake up in a mixed state of REM sleep and wakefulness. You are unable to move and it can be hard to breathe. You can experience auditory and visual hallucinations. They're almost always scary things like monsters, aliens, and people trying to either kidnap you or kill you. It only lasts a few seconds to a minute for it to wear off, and it isn't harmful, but it can be really frightening.