In that poor, sleepy haze of yours, are you remembering to burp her well overnight? Are you burping her in the same way as you do during waking hours, or are you doing a half-asleep diagonal-baby pat of some kind?
If she's not getting totally burped, she's not as full as she thinks she is.
I read another response that concerned me, about feeding her only every four hours. She is not waking for any other need than hunger at this age. Don't feed her less often than every 2-3 hours. "Getting her on a schedule," especially at this age, is dangerous business (it can also dry up your milk). Babies have such tiny stomachs. My nutritionist mom says as adults we shouldn't go more than 4-5 hours without at least a snack! So we should not expect a newborn to go four hours. Hydration also becomes an issue.
(On that note, it could be that she is hot and needing more fluids, thus the brief feedings - she could be using them to hydrate.)
Hang in there. You are so close to getting to that point where everything sorts itself out. Of course, there will be the appx. 6-week growth spurt...just be ready to spend all day nursing for a few days when that hits!
If you need more support, check out a local La Leche League meeting. They're so helpful. They also have a website, and I hear they have forums.
L.