These generally consist of behaviors occurring during REM sleep that normally happen while awake, or a disruption of normal processes during REM sleep. Sleep paralysis occurs when the normal paralysis present during REM persists into wakefulness. It is usually brief and resolves spontaneously, but can be frightening. Similarly, hypnic hallucinations are dream images that persist into wakefulness. Both can occur in normal people occasionally, particularly when sleep deprived, but also are common in narcolepsy. Cataplexy is the sudden onset of paralysis (as occurs in REM sleep) when the person is fully awake. For reasons that are not well understood, this seems to occur in the present of strong emotion (laughter or fear). This phenomenon also occurs in narcolepsy. Finally, REM sleep behavior disorder consists of the absence of normal paralysis during REM sleep. Because muscles remain functional, these patients can move about as they dream, sometimes leaving the room or (if the dream is frightening) having aggressive behavior that can result in injury.