For about the first 4 weeks post-op, I slept in a recliner, not because of chest/sternum/incision pain but because of stomach pain due to being so "stopped up" by the surgical and post-op pain meds. I was on a small dosage, but apparently my digestive system is very sensitive to these meds. At about week 4 I was re-admitted to the hospital for evaluation, as we didn't know why I felt so lousy. I was screened for endocarditis (thankfully not an issue), then I brought the docs to realize that my innards were not behaving. They administered a bottle of industrial strength laxative (magnesium citrate), which sorted it all out in very short order. After that, I was perfectly comfortable at home sleeping in my bed, even flat on my back. With all the complications, I don't know how soon I would have felt comfortable in a regular bed, but I'd bet it wouldn't have been long.
I think some of the bed/sleeping position issues may come from two things. First, how quickly you are able to regain normal body flexibility. I was lucky here - I had little loss of mobility as long as I moved slowly and carefully. So, in normal daily activities I was constantly stretching and flexing both muscles and my scar, so getting into and out of bed weren't a problem. Second, those of us who are "smaller" probably have an advantage here, as we are not moving as much body mass, and the "dangly bits" aren't as likely to cause pulling or strain. (I can't believe I said that. . . if Ross was still with us, he and I would have a 3-day jousting match with slightly off-color comments back and forth. Sorry for the digression, but I still miss him.)