why is my apple watch 5 shutting down in the middle of the night

Why is my apple watch 5 shutting down in the middle of the night?

The last 4 nights the watch has shut down, even tho it has lots of battery. I know the time (4:30a, 5:45a, 1:45a, 1:45a) because that is when my sleep app turns off.

When my IPhone alarm goes off, the watch doesn't buzz & I have a black screen. I put it on the charger and it shows a battery symbol in the upper left corner with a zig zagging line down the screen. After 1-2 minutes the regular charging screen pops up.