1) Suddenly AwakeVia: http://bunnysleeper.tumblr.com/
You wake up suddenly, but unlike this picture, you can’t move. You’re paralyzed.
2) All Your MightVia: http://kristenhead.blogspot.ca/
It takes all your might to finally move your body, as you scream out.
3) FolkloreVia: http://www.dia.org/
What’s happening is that your body is still asleep, while your mind is awake. Folklore said that there was a demon sitting on your chest.
4) The Logicvia: http://www.artmagick.com/
When you fall asleep, your body goes into a paralyzed state so you don’t thrash around and hurt yourself in your sleep. Sometimes your mind just wakes up first. Relax, and your body will catch up. Source