Your smoke alarm chirps at 4 because the house gets colder

Your smoke alarm runs on a nine-volt battery and on a threshold: below a certain voltage, the unit warns you with a short chirp once a minute. And batteries deliver less voltage when they get cold, so that warning almost always starts in the small hours, when the house is at its coldest.

1. A home usually hits its lowest temperature between three and six in the morning. A battery that held up during the day crosses the threshold at night, chirps, and by mid-morning, with the house warm again, goes quiet. That is why so many of these warnings look like a whim.

2. The chirp is short and widely spaced on purpose, so it annoys without draining what little is left. That is also why it is so hard to tell which room it comes from: the ear needs a continuous sound to place a source, and this one lasts under a second.

3. The sensor expires too. Optical units and ionisation units — the latter carry a minute amount of americium-241, less than a millionth of a gram — lose sensitivity with dust and with the years.

Tomorrow, take the unit down and look for the date printed on the back. If ten years have gone by since that date, replace the whole alarm; if not, change the battery and hold the test button until you hear the full siren, not just a click.