It took me several years of PAP therapy before I was able to. I was back sleeper, got fat and started down the apnea trail and lost the ability to sleep on my back - just couldn't go to sleep in a supine position.
I can now, most nights, and I feel SOOOOO much better the next day if I can. Back, hips and knees. Way more rested (I think arthritis and joint pain in other positions disturb my sleep).