All mothers are familiar with the childhood phenomenon of fullness in terms of meatballs and green beans juxtaposed with emptiness in terms of brownies. Mothers experience a similar phenomenon with regard to sleep. We combat a period of difficult sleepiness at some time of the day: around 7:45 when there's a baby in the house whose day begins at 5:12; at 10:30 when the children can tolerate no more maternal housework and the mother can tolerate no more maternal childwork (neither having been accomplished in any creditable way); in the early afternoon when younger children nap; in the late afternoon when they're all back up and crabbier than when they went down; in the early evening when they still can't be put to bed. During pregnancy this urgent exhaustion may strike several times a day, or persist without any break at all.
But then: they go to bed. And suddenly, the mother is awake! She feels great! She's like a normal person! She can do things again! So she does, far past the bedtime that should be her own, the bedtime that would keep tomorrow from feeling like miserable, somnambulant today. But she's not tired any more, and the bedtime of responsible mothers passes, and she enjoys hours of glorious free time instead of the 45 minutes she should have.
And then it's 5:12 again.