sleep paralysis is actually quite common. when we sleep, normally what happens is the body sends out a chemical to paralyze the body, because when we dream, the brain actually sends out signal as if the body's awake. So when we dream about running, the brain actually thinks the body is running, and tells our leg muscles to move. The chemical keeps us from actually moving. With sleep walkers, this chemical is insufficient, and they actually get up and walk around as in their dreams. With night terror, we snap out of our dreams too suddenly, even though the paralyzing chemical is still at work, so our minds are awake but the body is still "asleep". So we feel that desperate struggle to move.
The times I've experience that, mine were just as scary but not in the supernatural way. I'd wake up feeling like I'm smothered under my pillow, and I have to struggle with my body just to move the pillow off me, or I'd die of suffocation.