On my iphone 4s, there are these cellular data usages ranging from 1MB to 80 MB between 1 AM to 3 AM. This happens everyday. anybody knows what's going on and how to avoid it? During that time I am sleeping and the phone is connected to the WiFi.
Secondly I have noticed that when I lock my phone, it starts using cellular data instead of WiFi!!!
I have iOS 8.
Try checking your data usage history, as it seems like a known issue. A lot of people did complain but I can not find a solution to it.