I have one of the cocoons/papoose. Used it all the time as newborn. They are excellent.

Shell1: You are not supposed to leave them in for long periods of time as it is not good for their backs, I suppose you could look at it like us being kept curled up like that for a long period time, it would not be good for our backs, I have never tested the theory though . I don't really know of anyone who has had problems with their babies using them personally, my SIL is a Chiro so I went by her recommendation and also convenience for us in the end.