Z3 turns off while charging

Hey, just bought a new Z3, and I plug it in, at night before I go to sleep. In the morning after not waking up in time since the alarm clock didnt work, I was surprised to fins out that the phone is off, it has the black screen with the loadimg icon in the middle. Holding the power key turns it on normally. Heppened like 3 times now. When it is charging, but I use it for like a few hours nothing heppenes, its not turning off when I use it. And all kind of stamina and battery life savers are off. Dont have a fishy app or anything either.
Hopefully u couls help me with that.