Why is my iPhone battery dying when I'm not even using it?

Last night I went to sleep at 10 and my phone was fully charged. I wake up at 7:30 and my phone was dead. I didn't even use it. It just dyed. Just like that. Why?
I have an iPhone 5s iOS 9.1

How can you spell dyed like that? The phone did not change colour, did it?

Phones run things in the background like downloading or updating an app, receiving calls and text messages, so on so forth. To stop all these background activities I recommend shutting it down before going to bed and if you don't feel like shutting it down just let it charge overnight with the battery doctor app on and don't forget to turn your device off! The battery doctor app will keep things organized while you charge it overnight and make the battery as healthy as possible.

Try turning on airplane mode

Maybe it needs a new battery

Its died by the way.

As far as why, should check what apps are using the battery & how much of it is being used, though either way it shouldn't be draining as fast as that, but make note any battery for any device would go down regardless of being used or not.

Try restarting the phone.
Try resetting the phone.
Try restoring the phone to factory settings.

It's likely that you have apps running in the background. Double click the home button and swipe the apps out of the way and your iPhone will be more efficient with battery life. It happens to me all the time and I can make it through a day and a half without charging as long as I close the apps in the background.

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