After my bypass surgery I was told not to sleep on my stomach until I got approval from my surgeon. I was told the reason for this was to protect the sternum (breastbone) until it had a chance to mend after the surgery. Similarly I was not allowed to lift more than five pounds per hand or drive until cleared by the surgeon. Five weeks after my surgery my surgeon lifted all these restrictions. I have read that some bypass surgeries are now done without using a heart-lung machine, which I think may mean that the sternum is not damaged as it is when using a heart-lung machine. In any case, ask your surgeon and follow his orders.