On Monday night when I just couldn't settle ds2 after the feeds, my fatigue won out over common sense and I let him go to sleep with my boob in his mouth. When I woke and tried to get out of bed 3 blissful hours later my back was in screaming agony. Went to see a physio on Tuesday who told me that, by sleeping in an un-natural position without moving for several hours I had overstretched and damaged the muscles on one side of my back, and also done something to the lowest rib on that side. Several hideously expensive sessions of physio later I can just about get by with the aid of hot-water bottles and Ibuprofen (I really don't like take any medication while bfing, but have to bow to necessity here), but feeding, lifting, burping, changing are all AAAARGH!
So, be warned by my experience! Don't let the baby sleep with your boob in his or her mouth!