What causes sleep paralysis? what stages sleep is this in?

REM: Sleep paralysis is an overlap between rem and wake. More than a third of people will experience it during their life as a normal event. It could also be associated with narcolepsy and sleep apnea.

Sleep paralysis: Recurrent-isolated sleep paralysis: results in the inability to speak or move at the time of sleep onset or on waking. Diagnosis implies that narcolepsy is not also present and that no other disorders that could result in transient paralysis are present. Typically occurs during rem sleep.

Sleep Paralysis: Sleep researchers conclude that, in most cases, sleep paralysis is simply a sign that your body is not moving smoothly through the stages of sleep. Rarely is sleep paralysis linked to deep underlying psychiatric problems. http://www.webmd.com/sleep-disorders/guide/sleep-paralysis#1