My baby hates sleeping on her back. She's 6 weeks. It takes forever to get her to stay asleep when we first put her down if she's on her back. She'll either wake immediately or within 15 min. Once we finally get her to stay asleep she'll do 2 hours stretches. She loves tummy time so for naps we've been trying belly down with NO sheets blankets or anything. Not even a blanket to cover her. We just make sure she's dressed warm.  I do this for naps since I'm close and can make sure she doesn't get her head down where she can't breathe. She sleeps amazing. I want to try it at night but I've been terrified since it's not recommended til they can roll because of SIDS. But it's making my baby and me miserable with her sleeping on her back. She has had amazing neck strength since birth and can hold her head up while on belly (even if her arms aren't in front to support) for 5-10 min at a time. Even when we put her on her belly for naps she turns her head side to side to find a comfortable position like an adult would.