IPhone 5s
My i phone 5s shuts down automatically almost every night and in the morning i have to press both power butten and home button same time to switch on.whats the reason.
iPhone 5s, iOS 8.2
If the phone does that while connected to the adapter and charging, this could be hardware related and the battery has to be tested.
To make sure that this is not software related, set it back to factory settings, without using any backup data afterwards. Set up the rest of the personal settings manually and test the phone.
If it still does shut off while being charged, this is a hardware issue and the phone has to be serviced by Apple or an Authorized Apple Service Provider:
Use iTunes to restore your iOS device to factory settings - Apple Support