Most natural way to sleep?

What is the most natural- and by natural I mean instinctive, way for humans to sleep? Ancestral humans didn’t have all the comforts of spring mattresses or waterbeds, but they sure did have better backs than us! (At least I’m assuming they did)

What was their secret? I ask because I’m building myself a new room and when it comes time to get a bed (or not) I’d like to make a decision that will help me out in the long run and also let me tap into those good old ancestral roots!