John Gruber commented on using the Apple Watch overnight.

I think sleep tracking is an inevitable feature for Apple Watch. I’ve been wearing a Series 2 to sleep lately, and I wake up with between 55-65 percent battery remaining. I can usually get to a full charge — or close enough, like say 98 percent — just by charging it while I shower and get dressed. In my use, Series 2 does not need to charge overnight. So it might as well track my sleeping.

I’m using a Series 1 and it goes through the day draining about 80%. From experience, I can fully recharge the watch within an hour and a half. This means it is possible to wear the watch overnight as long as I can find a time slot to allow the watch to recharge before I sleep.