I know, I know, this is such a cliche. They even made an entire short-lived sitcom about it. It starred Will Arnett and Christina Applegate and was called Not Getting Any Sleep or Awake Because of the Baby or something. Sorry, it's hard to remember details when you've been up all night.
Dammit, it's on the tip of my tongue.
Anyway, you probably have the image of the baby waking up in the middle of the night crying, and this is what wakes up the new parents. Like it's all caused by the baby being unpredictable and disrupting this perfect schedule you want to set.
I don't know if a lot of people realize this, but even if everything goes right, it is impossible for a mom to get enough sleep under the recommended schedules. Even assuming the baby always goes to sleep immediately and sleeps as long as he wants, he needs to eat every two to three hours (for the first few months at least). If you're bottle-feeding, you can take turns with your spouse or nanny or undocumented indentured servant or whatever, but if you're breast-feeding, which all the doctors push pretty hard nowadays, that means every feeding requires the mom to be awake.
(Note: Apparently there is a big breast-feeding war on between two groups of angry people who think everyone who doesn't do it is murdering their baby or everyone who wants you to do it is the Gestapo. If you are on either of those sides, please don't hurt me, I am only a civilian passing through.)
"I don't want any trouble! Just take what you want!"
So if she's breast-feeding, in the best case scenario the mom sleeps no more than two hours at a time. She gets eight to 10 of these nap opportunities (napportunities?) in every 24-hour period. Three of them are lost because she has to eat. Maybe another one for a shower (once in a while at least). This is assuming the baby is a magical baby that goes to sleep immediately when he is supposed to and never cries. Since this is not the case, you can cross out a couple more. This leaves two to four two-hour napportunities, and due to stress and visitors and accidents and other factors, she could easily get less sleep in each two-hour window than the 90 minutes that experts say constitutes a full sleep cycle.
And if the baby should, by God's infinite grace, sleep more than three hours, just as you are starting to celebrate, medical professionals will tell you that you have to wake him up and feed him, because blood sugar, blah, blah, blah. You know that scene in The Shawshank Redemption where Andy gets out of the sewer pipe? This is like if someone pointed out he had to go back because he forgot his shoes.
It would be ironic because then Andy would have been guilty of murder.