Battery on the iPhone? [View all]

In addition to the charging cord that came with the phone, I also have a desktop square on which I place the phone and it charges it, Most of the time.

I don't like to just leave it on it for the night; it does not always reach 100% and last night before I retired, it was around 75%.

This morning it was dead. I had to use the cord to start charging it and after a few minutes it read 3%.

So I went to the settings, to battery Health, there is a graph and... it was more or less steady at 75% from 6:00 PM and then shortly after midnight it dropped at a 45 degree angle to about 10% at 3:00 am, and then proceeded, on a red lines to 0 by 6:00 am.

What would cause such a precipitous drop? How common is it?