From _David Smith, the creator of Pedometer++ (which I highly recommend) comes Sleep++ — an app for Apple Watch that monitors how you sleep at night based on your movement.

This obviously creates an issue since most people would charge their Apple Watch while they sleep. He offers this solution:

The TL/DR is to charge your Apple Watch in the morning while you get ready for your day (take a shower, get dressed, etc) and then again in the evening while you get ready for bed (brush teeth, put on pajamas, etc). Then put your Apple Watch in Airplane Mode while you sleep.

There you have it.