Once you have healed and your capsules are stable, usually around 4 to 6 weeks after surgery, you can start experimenting with different positions for sleep as you feel comfortable. Prior to that, it is best if you refrain from sleeping on your stomach and continue to sleep on your back or your sides. I also have my patients wear their postoperative bra during sleep for the first 4 to 6 weeks as well. It may still feel uncomfortable or unnatural for you to sleep on your stomach even at this point, but at least you can begin to try. The main reason for this precaution in my practice, in addition to purely comfort and the common sense avoidance of discomfort from something obvious like putting pressure on healing tissues, is to allow the implant capsules the chance to form naturally without any undue stresses from irregular or uneven directions which could potentially change the shape or position of the breasts. While there may be no evidence that this matters, it makes sense intuitively, and for something this simple and really risk-free to adhere to, I don't see any reason to tempt fate. The reality is that most girls who are habitual stomach sleepers will still probably find themselves on their tummies from time to time during the night, and I don't think that the occasional short time in this position is cause for alarm. However, being mindful of avoiding that position will help them limit this, and it is worth it in my opinion and probably helps limit the overall effect that that has. Good luck.