A Stupid Question?

I have been thinking about this for a few days now and could not come up with an answer. I haven't done any research yet on the matter, so when I do, it may sound like a stupid qestion to me.

During the day, my 4 mo. old daughter will take short naps usually and sometime will take an 2 hr long afternoon nap. At night when we put her to bed, she will sleep for a 7-8 hour stretch.

My question is: why does she sleep for that long of a stretch at night and only takes cat naps during the day?

In the day, no sooner will I put her down for a nap, after she falls asleep in my arms, and start to do some household chores that she wakes up crying. At night, when I do this, she sleeps till morning. Of course, I can't do any household chores at that time because I am so tired from my other 3 kids that I have to go to bed soon afterward.

Maybe the answer is self evident and I am just too blind or ignorant to see it. It really has no baring on how I do things, but just something that I have been thinking about.