iPhone 6 stops being able to connect to any network overnight

It's happened to me quite sometimes now, and it fixes simply by switching the phone off and on again. The thing is that I don't want to be doing that every time the phone stops being able to connect for whatever reason. I want to know whats going on.

It's always happened overnight. Before going to bed everything works well, and when I wake up, I simply can't connect at all, altough the wifi/4G sign shows the phone is connected.

I've tried switching the wifi or mobile net off and on; turning airplane mode on and off; changing wifi connection, and nothing works.

As soon as I restart the phone, everything comes to be normal again.