She sounds more like a nite owl.
I know this sounds kooky. But ... women tend to be insomniacs more than men. I don't recall the statistics but depending on how old she is, and of course the addition of children, women tend to want to nap during the day and are awake more at night.
A lot of women also take naps pretty much during lunch breaks or mid afternoon, sometimes at a desk, or sitting on a stool in a lab somewhere. I've read about this phenomena where it just happens to some not all.
Maybe your wife is like that. Sex at night is just something that entertains her. But she's a day-napper. Ha ha ... okay seriously.
The other thing is, some people get energetic after sex. Its like driving for hours at a time. You may be tired but then you're wired. You can't sleep, and you have to keep moving, even if you're exhausted.
Don't sweat it. Its just her rhythm