Smoke Detectors: Why do batteries only fail during the night?

I am convinced that somewhere in the world there is a engineer just laughing his head off over the design of smoke detectors and their batteries. Why is it that the low battery warning beep only occurs during the night? Last night, or should I say this morning, at 3:08AM, that familiar beep woke both my wife and I. I heard it, wanted to pretend I didn't hear it, but a minute later my wife said "did you hear that beep?" Yes I did. That meant that it would sound again in 15-20 minutes. And that also meant that I would need to get up and figure out which of the 5 smoke detectors on the upper floor was the culprit. And at 3:22AM, another beep. Since two are in the kids rooms with doors closed, I was able to "quickly" (how quickly can you be at 3:22AM) ruled them out. So, I got up and stood in the hallway, locating myself between the other 3, hoping it wasn't the one in our room, with the vaulted ceiling, and waited. Next beep came at 3:34 and I determined it was the one in the hallway. Got the step stool out and removed it from the ceiling, pulled the battery, and sent it to the basement since it continued to beep after the battery was removed. (Capacitor still had a charge in it.)

Two weeks from now we end Daylight Savings Time, when I would normally go through the house and replace all of the batteries. I think smoke detectors know this as well.