“Underscore” David Smith has a new sleep-tracking Apple Watch app, and actually figured out a reliable routine to keep it charged:
Today I released a new app Sleep++ that uses the motion tracking capabilities of your Apple Watch to monitor how well you are sleeping at night. This is one of the capabilities made possible by the improvements of watchOS 2.
To use the app you need to wear your Apple Watch while you sleep each night. This presents an obvious problem, when do I charge it?.
The method is similar to what CGP Grey theorized in Cortex 3. Whether this can work for you will depend on how much you discharge your Watch each day — owners of the 42mm model who don’t use workout mode for very long each day will have the easiest time.