I normally fall asleep on my back or my stomach. There's the rare occasion where I sleep on my side, which is quite strange, because that was the only way I used to sleep... Of course, that was because when I was much younger I was afraid that there was someone watching me from the side of my bed that did not face the wall. I had an avid imagination, if that clears anything up. But as I grew beyond that, my territories expanded.
You see, my father told me a story about a caveman that ate the toes of children while they slept. That scared me enough to make me keep myself curled up in sleep until I was about nine.
As I grew, though, I slowly let my feet extend across the bed, gradually becoming daring enough to let my feet reach the end of the bed. And now, I sleep either on my stomach with one hand under the pillows, or on my back, with the pillows just under my bed.
Sometimes I'll wake up facing the opposite direction with the pillows under my head... And if worse comes to worse, once in a while, I find myself in the crevice between my bed and the wall.
The worst, though, is when I wake up with a terribly sore neck. I'm not sure what causes that, because when I wake up like that, I'm normally in a comfortable position.