Why Does Your Phone Battery Drain Overnight?
A dark screen does not mean the phone has stopped working. Notifications, syncing and other background activity can continue while you sleep.
The useful part
Network conditions and battery age can also affect the result. One unusually bad night does not identify the cause; a recent update may temporarily increase activity.
Try this
Check Settings → Battery → Battery usage in the morning. Look for an unexpected app, update it, and compare another night before changing several settings at once. Restricting background activity can delay notifications.