Ah, sleeping! Glad to see so many different experiences here.
I have never been able to sleep on my back. Only my side or stomach. When I was 16 I was in hospital in a body cast with a broken pelvis (freight elevator mishap). I was flat on my back for 2 weeks and did not sleep a wink until I could flip myself over. All my life I've been a restless sleeper. However, when I was in the hospital with my hip, I slept through the night on my back in what must have been the first time in my life, probably due to the morphine. Second night also, then started alternating between my back and my side.
Once home, I was sleeping alternately on my back and side. However, for the first couple of weeks I was literally getting up every hour to go to the bathroom, maybe because of the pain meds and blood thinner. Now I get up once, sometimes twice, and I end up sleeping on my side again, sometimes even the operated side. I try to keep a pillow between my knees whether on my back or side, but it usually goes astray. I sometimes even end up on my stomach.