The sensation that there's someone in a room with you is called an “intruder” hallucination, according to an article published in Frontiers in Human Neuroscience . The authors say this may happen when someone is experiencing a glitch in their mirror neurons, which fire off when you move or when you watch another person move. During sleep paralysis, activity in these neurons could make you think you're catching sight of someone or something moving,when you're really not.