Usually Normal. Most patients who are doing well after breast augmentation surgery, and do not have evidence of abnormal scar formation (encapsulation) do very well sleeping on their stomachs. Of course, patients with very large breast implants may have more of a problem doing so.
Natural if soft. The sensation of sleeping on your stomach will greatly depend on several factors such as: the size of your natural breast, the size of the implant, the degree of ptosis, and the amount of capsular contracture. A modest sized implant in a moderately sized breast with mild ptosis and grade I baker capsular contracture will feel quite normal and comparable to natural breast tissue.
Reportedly ok. Women with saline breast implants that are proportionate to their breast structure and do not have a capsular contracture feel comfortable sleeping on their tummy after one to two months. Some women with hardening of the tissues around implants (capsular contracture) may not be completely comfortable and in that case they should seek evaluation by an expert.
Breast Implant Feel. Initially it will be quite uncomfortable and is not recommended. After you are completely healed, they should be able to withstand sleeping on your stomach. How strange it feels will depend on the stiffness of your scar tissue and the size of the implant. Talking to other patients can give you an idea of what it will feel like but ultimately it is a very individual experience.