Willow, I did research on this when DS was little.

When a mother laughs, it produces "sleep through" hormones into her milk. This sleepy milk is released around dusk. So laughter makes a baby sleep better. (I prescribed myself half an hour of Monty Python a day after that - there was a slight improvement. But if I got 2 minutes of laughter a day that was good.) However, when a mother has depression, she doesn't laugh. Therefore she doesn't make sleepy milk. Therefore the baby doesn't sleep. It's a vicious cycle.

Given that few adults, even those without PND, don't laugh for a half-hour a day minimum, sleeping through shouldn't really be an expectation in younger babies.