Sleep paralysis, which occurs at night or early morning awakenings, is the inability to move any part of the body except the ocular and respiratory muscles, which may or may not be associated with hallucinations. In the first episodes there is an intense fear because the reversibility of the condition is not yet known; it lasts a few seconds and it often stops via external stimulation. It is characteristic of narcolepsia, but it may be an isolated symptom, in which case it is activated by sleep deprivation, irregular schedules, stress or drugs. In individuals with a history of sexual assault there is a higher prevalence of sleep paralysis. Although it does not require special investigations, polysomnography does show a dissociated state with alpha activity and muscular weakness.