Battery drain over night: an experiment

THere are lots of threads on battery drain but because I was having frequent trouble since the previous updates (to 8.1, 8.2 and 8.3), I decided to run a simple experiment. Every night for the past couple of weeks, I set my iphone 5s to Airplane mode. No wifi, no bluetooth, no cellular data activated. I use iCloud sync only for contacts and calendar. All apps were closed. With these settings identical every night, I checked battery usage each morning.

Battery usage varied from 2% to 21% apparently at random. It didn't depend on day of the week or on the apps used the evening before. I had the impression that battery drain was stronger when it was below 30% charge than when it was above 70%, but I don't have enough data to be sure.

What can I do to prevent this random battery drain? Does anyone know what could cause this?