Charging Apple Watch Series 2 overnight?

No it doesn’t. You could think of technical reasons why that would be worse for a battery, but the Watch itself is pretty good a managing the charging process in order to keep the battery in good condition.

 

I’ve been doing it for 3 years, and it’ll still run 2 days on a charge.

 

Nowadays the used battery on different devices are thought to be used both overnights and in each moment you need. No matter if your battery is dried or almost full.

 

Consider Apple Watch models that you see in stores, those charge 18 hours a day/7 days a week on an annual basis. Your watch will be perfectly fine charging overnight.

 

The Apple Watch has a standard "nightstand" feature, pretty sure if leaving it on charge overnight was problematic then Apple wouldn't have provided this functionality.

 

It used to be that it was always best to run a battery all the way as close to empty as possible before charging back to full was what was best to extend the life of the battery. But that hasn't been the case for many years now. Charge whenever it's convenient for you.