This doesn't happen all the time, only sometimes.
Within 7 hours, battery went from 95per cent to 0. In this time, phone was not used at all, no apps were running. Usually, in this time (overnight), it'll go down to like 70 per cent or so. Happens once every few days. Even with heavy use during the day, the battery doesn't drain so fast.
What the hell is going on with my phone?
It's a Samsung Galaxy s4.
Read more:
What's wrong with my Android's battery?