I will often take my watch off an hour before bedtime and put it on the charger until I go to bed...and shut it off overnight. If the watch stops charging once it's at 100%, I'd love to know why it is often warm after an overnight charge that should have taken only a couple of hours.
The watch stops charging at 100% and will cool down and continue to run on battery power. When the level drops to 99 or 98, the charger will kick back on briefly to charge it back up to 100 which will warm it up again. This is why it will sometimes be warm in the morning.