"I slept like a baby"

That saying makes absolutely NO sense ... unless you mean to say that you slept intermittently for very brief periods of time at the exact opposite time of day that the rest of the world was sleeping. If you use it in that sense, then I guess the saying is perfectly accurate and acceptable to use. However, any usage of this saying to imply that you had a nice, restful sleep at night is just plain inaccurate and, quite frankly, it is just downright mean if you continue misusing the phrase -- you are giving new parents around the world the false impression that babies are good sleepers.