What are the odds?

A few types of mechanical devices are the bane of my existence. They exist to torment the Phactor. The largest category is anything powered by a small gasoline engine. With only one exception, they are a constant bother working with an inverse reliability to your needs. But the data points are building up, and presently the score is 9 to nil. Why do all smoke/carbon dioxide alarms sound their battery or instrument failures between 1 and 3 AM? All things being equal shouldn't they die with equal frequency during all hours of the day? Of course at a civil hour the blasted things would not jar you from a sound sleep with an adrenaline-induced heart attack. And then after the situation is put under control by ripping the innards out of the offending device, she who sleeps like a log mutters, "did something happen?" They do not make an alarm loud enough or persistent enough to alert some people who would die in their sleep if there really was an emergency and they did not have an ever vigilant partner looking out for them. And the bed cats are no help at all going from panic mode to are-you-going-to-feed-me-? mode as the situation is resolved. So what are the odds this happens by chance?