Battery Low

Murphy’s Law — Smoke Detector Division:
You sleep at most 1/3 of each day, yet the chance of a smoke alarm starting to chirp due to low battery during sleep time is 99.99 percent.
Corollary: If you get up to deal with it, you’ll find you happen to be out of 9V batteries.

And if you don’t get up to deal with it, you’re not getting back to sleep no matter what.

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