Bedroom Shenanegans

Bit of information for the ladies out there. I am great in bed! Once I'm asleep I'm out like a light until my alarm clock wakes me! Or is that not what that's supposed to mean?
I don't know, I never get the reasoning behind sleep related phrases. For example what is it to "sleep like a baby"? Does it require waking up fourteen times with a full nappy? When you "sleep like a log" do you wake up covered in moss? Why would you "hit the hay"? or the sack for that matter. Has anyone ever seen a "wooden hill"?
Nothing but questions, never any answers.