Sleep paralysis (aka "old hag") usually occurs between the state of sleep and walking up. Do you get a heavy weight pressing down on your chest? If so do you feel as though you can not breathe?
There are two (major) views on this - I googled it and read alot of research, about an hours worth so im just giving you a quick over view -
it is thought that these experiences are most likely cause by the body and the brain being out of 'sync' when alseep. Normally during rem (rem is also necessary to supply the cornea with oxygen. Delta
sleep, associated with large, slow brain waves, is what most people call the deepest kind of sleep, when you are hardest to awaken. It is necessary for the brain/body to rest and heal.) sleep, the body is paralyzed - if it wasnt then you would be acting out your dreams - because you are paralyzed during this cycle of sleep your brain much 'flip a switch' to bring you out of this state. Now all the pages I read stated that the 'scientific' reason for this is because your brain hasnt yet flipped the switch. Then they all went into the percents, ever 2 people out of 10 suffer from this condition.....Kinda weird huh?
Hypnopompic paralysis what its called :d and is often associated with narcalespy (sp).
Some people say that when they wake up there is another presence in the room, an 'evil being' . This is another belief, that the sleep paralysis has something to do with the evil spirit that is often there, however, the scientists have proven that during sleep paralysis a person tends to hallucinate. :d
hope this helps you
winters gate