As someone who has been experiencing sleep paralysis for years now, I would say It's much more of a sudden fear. One that makes the hairs on the back of your neck stand up ever so slowly. Not know if the thing you see in the corner of your eye is really there or not, and the inability to move or even turn your head won't help as the thing gets closer to you. the only thing you can do is move your eye and make grunting sounds. The only thing that snaps you out of it is that fear threshold reaching its maximum as you gasp for air scanning every part of your room for any resemblance of the figure you thought... or you felt standing right beside you...