Funny how you can get used to sleeping on your back, even if you've never done it before. I used to be a side sleeper, and I am slowly becoming one again but I go really sick a couple of years ago and was forced to sleep on my back in hospital. When I got out, I couldn't seem to sleep on my side at all anymore. I had become a backsleeper.
There is something in the bible about being on your back to dream. It's a foggy memory . . . maybe David (?) "lay himself down to dream". It seems fairly universal that being on your back is the best way to have a lucid dream.