I suffer from sleep paralysis. I only get it if I sleep on my back (I never purposely sleep on my back)
I first had it around 8 years ago. At that time I was sleeping alone and could manage to bring myself out of it by concentrating really hard on trying to move a finger/hand. Only had it a few times and then no problems for the next 7years.
Last year it came back and I've been unable to snap myself out of it. This leads to me panicing and struggling for breath. Initially my weird breathing would be enough to wake my partner and her voice would bring me round. Then she needed to tap me, now she needs to really shove me to snap me out of it. Can't be very nice for her...I've started funny breathing/making groaning noises to try and wake her so she can snap me out of it.
I've read about it and apparently it's something to do with entering/leaving REM out of sync. Apparently the doc's can give you something to stop you entering REM sleep but I don't feel it happens to me regularly enough at the moment. I guess I suffer from it once or two every few months.
In my case I think it's probably related to stress.