A car battery dying overnight can be caused by several factors but first thing I would do is check to make sure your interior light is shutting off when you close the door.
I'm talking about the light above you in the car that comes on when you open the door.
I had a bad door switch that wouldn't shut the overhead light off inside my car and it was causing my battery to go dead overnight.
If it's not that then make sure you have not left anything such as a cell phone or anything else plugged into the 12 volt outlet even though some cars are supposed to shut that 12 volt outlet off when you turn the car off some cars don't.
If none of that is the problem, then your alternator could be bad or have a bad diode in the alternator causing a short that is draining your battery overnight.
Corroded battery cables can also cause a battery to not fully charge and could cause a dead battery too.
Also how old is your battery? If the battery is 5 years old or older then it could be going bad and need replaced.
I would take the car to an auto shop or to Autozone and have them do a battery load test and alternator test to find out what the problem is.
Some other electrical problem between the starter and battery could also be causing the battery to die overnight too.