It is a sleep disorder. No one knows the real cause but there has been some research into it. From my understanding when we go to sleep we release a chemical that ‘paralyses’ us so that we don’t act out our dreams and cause harm to ourselves (it’s actually this chemical that sleep walkers don’t produce enough of hence why they walk when the sleep.) This chemical is rapidly broken down when we wake up and the ‘paralysis’ is released. Basically, with sleep paralysis the chemical isn’t broken down quickly enough so we become mentally awake and alert but we can’t move our body as the chemical is preventing us from doing so. I hope that makes sense. It is a pretty scary experience but it could be worse, some people who suffer from sleep paralysis also experience hypnagogic hallucinations. Can you imagine being unable to move and then having some wacked out hallucinations as well? 