When we dream, we have many thoughts and dreams. For our own protection, our brain closes the connection between itself and the rest of the body, so that, even if you move your body image while dreaming, your real body stays still and safe. That feeling of not beign able to move your body, both when randomly wakign up in the night, or when WILDing, is called Sleep Paralysis.

You know hypnic jerks? Those short dreams you have when extremely tired, of which you wake up feeling really freaked out? They happen because you dream while not on SP, so you actually move your body while dreaming. That, of course, makes you wake up.