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

I am 5 days post-op from my breast reduction and liposuction of the lateral chest wall. So far, when I sleep, I sleep in a seated position, propped up by pillows. I'm finding that sleeping like this is causing my butt/tailbone area to be really sore. I know I'm not ready yet to sleep on my side, however I think that if I were able to lay flat on my back without all those pillows I would probably sleep better. How long should I wait before sleeping flat on my back? Is it safe at this point?