Fast unloading battery after iOS 7 update?

Hello guys,

2 weeks ago, I still had iOS 5.something on my iPhone 4 because I never took time to update the software, and also I preferred the old design. However, in order to use an app (star wars: tiny death star), I decided to update my Iphone to iOS 7.

Since the update, ma battery does not last as long as before, or even does not last at all. I did not change my habits (iPhone remains all day in Flightmode, only with wifi on, I do not allow a lot of pushes, almost no localization) but when I go to sleep with a full battery, it is completely empty next morning. It did not happen before !

So I was wondering why did it happen? At the beginning, I was suspecting my app: star wars: tiny death star, because it works background (you earn credits, you need time to build new levels etc), but now I'm thinking it's maybe iOS 7

What do you guys think? Is there any solution to get my old battery life? Don't hesitate to ask more details.

Thank you for your help !

(PS: sorry for my English, I'm French)