Sleeping Positions

Liz • I'm 31, love traveling, scuba diving, and enjoy doing yoga and sports. Currently expecting our 1st baby on December 24!
I'm normally a stomach sleeper & I know you're supposed to be sleeping on your left side (to promote circulation etc). 
I have a pregnancy pillow and start the night sleeping on my left side, but when I wake up in the middle of the night I find myself sprawled out on my back. Has anyone else experienced this?