The short answer: Because your baby isn't tired.
"But," you say to me, "He was tired at this time last night and it's been more than four hours since his last nap. He should be tired."
He's a baby, not a clock.
Maybe that sounds harsh. I do not intend it to be. I'm just trying to save you some heartache.
When my son was a baby, I read all that wonderful research about sleep training and not letting the baby sleep too long and trying to put him on a schedule to understand the difference between night and day.
And then, I realized my son was a bear. And as a bear he would stay asleep for a really long time. And that those books relied on averages. And that everyone's circadian rhythms are different. And, most importantly, that he was not a clock.
Most new parents do not realize that a baby's sleep patterns vary within the first few years of life. Drastically. Just because your baby was sleeping through the night for a week doesn't mean that it will continue into next week. (I know that is depressing. I'm sorry.)
But there is hope. Eventually, their sleep pattern emerges. And then you only occasionally run into problems. Like the time my four year old left his bedroom at 11 pm to come talk to me because he was bored. Clearly, he wasn't tired. And after laying in bed for three hours he had had enough. But, I rocked him a bit and put him back to bed anyway. I reminded him that his body needed rest (and so did mine). He was groggy in the morning (more so than normal), but the next night he slept better.
Is your sleep pattern normal or are you all over the charts in how much sleep you get at night? Share with me in the comments.