Why...?

Why is it that smoke detectors always tell you their batteries are low in the middle of the night? OK, so we should have changed them out back in the spring, but we just got busy and forgot (like a lot of people I think).
I just don't understand why that annoying little "chirping" sound couldn't go off in the middle of the day when I'm wide awake. When I could go buy a battery and get it installed before bedtime. Instead, it happens around 2 a.m. when we're in a deep sleep. We both hear the chirp, and Randy immediately says "OH NO" with a groan. Soooo, we get up and walk around downstairs waiting for another beep so we can figure out which detector it is. It beeps again, but nope it wasn't the detector in the hallway. We wait for another beep...and follow it to one in the living room....of course, if just happens to be one that's up really high. Randy grabs his house shoes and trudges out to the detached garage for a ladder and brings it into the house (running into walls with the ladder because it's so big, not to mention the fact that he's half asleep and he still has vision problems. Not truly wanting to, I do the right thing and offer to climb up to grab the offensive detector. Randy thinks he can do it though, so up he goes...still half asleep and on wobbly legs. He had to climb to the very top of the ladder (really glad *I* didn't have to climb up now), and turns the beeping plastic thing. It's a little hard to budge, so being the ever so impatient person he is, dh practically rips the thing off the wall. Once he finally got the wires disconnected, he took the beeping carcass and plunked it out into the garage and went back to bed.
Guess I'm going shopping for lots of batteries today.