Smoke Detector Battery Replacement Time

I wonder why it is that the smoke detector battery low indicator (a harsh beep about every thirty seconds or so) always goes off in the middle of the night. Either right after you are settled in and are in that perfect REM state or a couple hours before you get that right amount of sleep.

We usually change out the batteries in our smoke detectors each year. If we remember, right around the time we go off daylight savings time. But if not, right after that annoying battery low indicator tells...