How long after breast reduction surgery until I am able to sleep flat on my back?

At this point after surgery, you should theoretically be fine to sleep flat if that is more comfortable for you.  Since I didn't do your surgery, I would tell you the best plan is to call your surgeon's office to ask what their recommendations are. I would guess they will tell you the same thing.  Good luck with your recovery!

Congratulations on having undergone the breast reduction procedure. Your plastic surgeon will always be your best resource when it comes to providing you with postoperative instructions/advice. In my practice, most patients feel comfortable, lying completely supine 5 to 7 days after the procedure is performed. Best wishes for  an outcome that you will be very pleased with.