A At the moment, there aren't studies indicating that prolong sitting affects Brazilian butt lift results.
In my personal opinion, I do not believe that prolong sitting affects Brazilian butt lift results. I have many patients who have combination procedures, such as a tummy tuck and bbl and do not have an alternative position to sleep in and have maintain an excellent result. I do not restrict my patients who have this procedure, from sleeping on their buttocks.
It all comes down to the preference of your plastic surgeon. When selecting a plastic surgeon, please always search for a Board Certified Plastic Surgeon.
Best of luck...Dr. Sozer