My son's exclusively breastfed (we have introduced solids at night in the last week or so) and he sleeps GREAT at night. Naps are a different story all together...we're seriously battling short naps and have been since he was about 10 weeks (he's 19 weeks now) I think the whole formula babies sleep better theory comes from the fact that formula babies DO usually sleep longer stretches before a breastfed baby does, but I'm pretty sure it all evens out after about 6-8 weeks. I've tried giving Seth cereal in the am before his first nap to see if it will extend it and so far, no luck, so we don't worry about the solids in the morning yet. Do what works for you and what you feel comfortable with. For me, our major incentive to keep on breastfeeding is it's FREE!!!