Night Terror
Becoming mentally aware before the body "wakes up" from its paralyzed state can be a terrifying experience, as people realize they can't move or speak. Frequently, these episodes are accompanied by hallucinations and the sensation of breathlessness. Such hallucinations likely gave rise to the myths of the incubus and the succubus, demons that pin people down in their sleep (and sometimes have s*x with them).
People may also sense a malevolent presence nearby or believe they are about to die. Some sleep paralysis episodes come with feelings of falling, floating or dislocating from the body.
