David Smith (also known as “Underscore”) dropped Sleep++, an Apple Watch app that allows you to track your sleep a la Fitbit and others. The key here is needing to keep your watch on all the time. So how do you do it?
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.
Read more about this process over on his blog.