Sleep paralysis is a horrifying prospect at the best of times, but did you know that your body undergoes a different form of muscle freeze as you slumber? Don't worry though, this is entirely normal and not life threatening in the slightest. You'll barely even notice, until you wake up with a bad back again that is.
During the REM (Rapid Eye Movement) cycle, your muscles freeze up completely, disabling any ability for movement. In effect, you become completely rigid and can't smack the lucky partner sleeping beside you. They'll thank sleep for this, because it saves them the black eye and you the menacing glare each morning.