Why 4:00 a.m.?

The other night, I was awakened about 4:00 a.m. by a chirping smoke alarm. Not blaring--as in there is smoke, therefore, a fire, so you better get up RIGHT NOW!--but just the annoying and persistent chirp of the smoke alarm letting you know it needs a new battery. Since there are eight smoke alarms in my parent's house, and they said they all chirp if one needs a battery change, I decided to wait for the parents to be woken up by one of the upstairs alarms to help me figure out what to do. (I don't know where the batteries are in their house--what if the one needing a replacement was the one on the vaulted ceiling that I can't reach and I don't know where their ladder is, etc.) Well, after about four chirps, it stopped and I gratefully went back to sleep. The next morning I told my parents, who agreed with me that it was strange that it had chirped and then stopped. So life went on. Exactly 48 hours later (so, again, at 4:00 a.m.) the smoke alarm started chirping again. This time I got up and went to stand under a smoke detector upstairs to see if they were chirping--they weren't--which explains why my parents didn't wake up two nights ago, and why they weren't waking up this time either. So I went back downstairs to see if all of the ones in the basement were chirping by standing under each one. This all took awhile because the offending alarm would suddenly stop chirping for several minutes at a time. In fact, I went back to bed at one point because it had been so long with no chirping. Of course, about 2 minutes later, the chirping began again. By this time, my mom was awake--not because of the smoke alarm but just because she frequently wakes up in the middle of the night and can't go back to sleep. So she helped me locate a battery, we finally determined which one was chirping, and I changed the battery. Thankfully, it worked--no more chirping. By about 5:15 I was back asleep. So my question is...why does it always have to be at 4:00 a.m.? In Arizona, our smoke alarm also started chirping in the middle of the night. I had to wake my husband up (only I am awoken by small chirping noises--I'm a light sleeper) and send him to get a ladder out of the garage while we tried to determine which alarm needed attention--ours were all chirping together so you couldn't tell which one needed the new battery. We replaced the batteries in about three before the chirping stopped. We decided to do the rest of the detectors in the morning since another of the ones we didn't change would probably need a new battery soon and we didn't want to repeat this middle-of-the-night adventure. The problem was, by the next morning we couldn't remember which ones we had fixed in the middle of the night and which ones we hadn't. So we left it--hoping the next time the chirping would start during the day. No such luck--a few weeks later we were again woken up in the middle of the night by a chirping alarm. I have to wonder if some sick mind at the smoke alarm factory somehow built the things to ensure that batteries always hit the low level in the middle of the night! It seems to be the same way with childbirth. All four of my babies--when they arrived on this planet--either kept me up all night or woke me up in the very early hours of the morning. They still have a tendency to do that.