iOS 9.3.1 - battery dies faster

hello! i just upgrade my iphone 5S with the new version of ios 9.3.1 and i have noticed that my battery dies faster than before. and i'll give one big example: during night, when i'm sleeping and the phone is sleeping too 🙂 (about 7-8 hours) Before upgarde to 9.3.1, my battery goes from 80% to 79%. Now After upgrade to 9.3.1 goes from 80 to 75. why?? i can't understand why!?