As others have noted, it's not an ideal situation to leave batteries on a charger overnight or unattended. Attended doesn't mean watching them constantly, just being around and checking on them occasionally is all. But with that said, modern "smart" chargers basically shut down once the battery(s) reach full charge to prevent overcharging and the related hazards. So likely won't hurt your batteries. However, you have to keep in mind that the chargers and batteries are made by humans, humans screw up, also electronics can malfunction at any time. If your comfortable with the risk, you won't be hurting your batteries any. Personally, I unplug the charger before leaving the house or going to bed, and I try to charge batteries when I know I'm going to be around and aware. I've forgotten some times, as has anyone else, and had nothing happen. But I'm not comfortable with the risk of doing it on purpose. Your call of course.