Well, according to Dr. Marilyn Moffat, a professor of Physical therapy at New York University, your sleeping posture probably isn't to blame. More than likely, you strained your neck the day before maybe by cradling the phone between your neck and shoulder, or twisting too quickly to look out the car window--And by lying still for 8 hours, you gave it time to cramp up.
So here's a quick remedy for getting rid of that obnoxious crick in your neck, courtesy of Dr. Moffat. We found this is Good Housekeeping.
First take a hot shower to loosen things up. Then lie on your back with your knees up, feet flat on the floor. Tighten your stomach and tuck in your chin. While in this position, turn your head from right to left, five times in each direction.
This should help get rid of the kink, and you'll be doing cartwheels in no time! Then tomorrow you can do it again to get rid of the cartwheel pain.