About five or six, maybe even seven years ago I would fall asleep in any position, shift many times, wake up in some random position as well. Then I got interested in lucid dreaming and astral projections. One thing mentioned before doing these, was to get yourself into comfortable position. Then relax your body, feel it shutting down while keeping your consciousness aware. To do so, I found that I can only begin relaxing if I am lying on my back. With an idea in mind that if astral projection, lucid dreaming happens, I may easily get up.
However, I would usually try this before my night sleep, being tired already. But I still could get into the relaxed phase. And for my great surprise, I would find myself fully rested and remembering lucid, vivid dreams. I loved it. So ever since then I always fall asleep on my back and I always consciously relax my body. Also, I sleep naked.
These past few months I find myself waking up in random position, be it on back, on stomach, or either side.
Oh, and I always have to be covered with something. For example, if I had to sleep in a train station on a bench, I would obviously be dressed and would probably even want to wear my jacket. Sometime during the night I would wake up shivering out of cold. But if I cover myself with same jacket (as a blanket), I can sleep through. I think when being covered with something, the air around me gets warm because of my body and creates sort of a warm environment. And I just can't stand cold.