Life is so starkly different after having a child that parents have clear recollections of things they "took for granted" before and feel overwhelming compulsions to share it with expecting parents. I, myself, have been guilty of this. The one I heard most often was, "get some sleep now because you'll never sleep again." Now being the lover I sleep I am, I did take their advice and rested as often as I wanted during my pregnancy and made sure I was well-rested every night especially in the end just in case this was the night. I knew and believed that there would be changes in my sleep, but never did I fathom how much my relationship with sleep would change.
Infants sleep whenever they please. You may be under the false impression that you can create a schedule. Infants sleep every few hours and if your lucky, those hours will coincide with "normal" sleeping hours. They decide when they sleep not you. Your introduction to this new sleep pattern is like being dropped into ice water... it quite a shock to the system, you might get used to the change in temperature over time, but your body will not be as effective because the change put you in survival mode. Your emotions become heightened, you can't seem to remember the littlest task, you literally feel yourself going insane... now, add in the home and parenting responsibilities you must juggle all the while not seeming utterly insane to the outside world.
No wonder most mother's have a uniform of sweat pants and hair in a constant pony tail. We do not have the mental energy to pick out an outfit or set our hair besides its only a matter of time before someone with ketchup hands decides to hug me or vomit on me. So, when I hear a childless person say, I stayed in bed all day... I can't help myself, but to say, "enjoy it now because one day, you'll miss it."