My surgeon recommends waiting until 6 weeks before sleeping on the operated side. He uses a posterior-lateral approach with the 90 degree restrictions.
It's important to check with your surgeon about things like this because it depends so much on what surgical approach you had and other little things that are individual to your surgery. For example, some surgeons want partial weight bearing for uncemented and others say full weight bearing is OK from the get go, as mine said. I also was given permission to do more things than many other surgeons in my area, but lying on the operative side is on the "wait till 6 weeks" list.
I am a little more than 4 weeks post op and for about a week, I've been lying on my unoperated side with a big king sized pillow between my legs for part of the night, but I get a little nervous that my leg will slide as I sleep and seem to gravitate back to my back for the majority of my sleep, just to be on the safe side. It does feel good, though, to just lie on my side for a while and takes pressure off my back.
Although I'm a natural side sleeper, I've kind of gotten used to sleeping on my back for the past 4 weeks. I use lots of pillows so I'm not so flat and sometimes put a pillow under my knees if my back is bugging me.
Dorothy