I’ve had one Total Reverse Replacement and it took about a year before I could even think of sleeping on the affected side. Stomach sleeping would probably require too much arm extension and I stayed on my back, retraining how I slept, because it just hurt too much for me. This was one of my toughest joint replacements and took the longest to rehab but I heartily recommend PT as long as you need it. It is the only way I managed with my Reverse Total Shoulder, which now 18 months later, is way better than pre surgery. The pre surgery pain is gone and I am glad as that was pretty horrific.