That must be a frightening situation, I remember reading about several cases some years ago on the internet. I don't believe that any more progress has been made into the actual cause but it seems that it's more common than once thought.
I have something similar and I hate it. I am awake but my body is paralyzed. I can't move and so I am not able to wake up. I just lie there thinking and trying to move something, anything and it won't. It's scary and weird. I know there is a drug that will do that. Your mind is working but the body doesn't. Hope we can find an answer to this. Ally
After looking through the internet, this is a very common thing. Mine began after an incident of waking up unable to breathe. It felt like someone had their hand over my nose and mouth and I was running around in a panic, unable to draw in any air. After a few attempts, I managed to gasp some air into my lungs and everything returned to normal. I've never had an episode like that again, but now get the shivering.
I thought I was the only one who experienced this. Some mornings I wake up and it's like my body is shivering inside, like you are cold. Only, your muscles aren't moving.
It does disappear quickly though, usually within about 20-30 seconds when I open my eyes. I'm wondering if it's the blood vessels opening or something from low pressure when asleep. It doesn't bother me, but it does feel a bit odd. It seems to be from the top of the chest to the bottom of the rib cage and doesn't go down my arms/legs or tummy area. If anyone knows what this is, I'd also be very interested.