Under Battery Usage, tap on the clock icon at the right which will show on screen and background time. This will give you some idea of which app is consuming battery in background. When you lock your phone, just make sure you lock it when it shows home screen. If you had an app running on the screen while locking the device, it will continue to run. Sometimes, app might be stuck and continue to use battery in background. Before you sleep, you may try to clear all apps from the app switcher and check in the morning.