Since upgrading to iOS 6 my phone has used large amount of data between 12 and 1 am, usually somewhere between 100-1000mb. What is my phone doing while I am sleeping to use up all of my data?
Chances are, it isn't and that that's the time of the day when your carrier tallies and counts some of your day's usage.
iCloud can only backup through WiFi. So it wouldn't be using your cellular data nevertheless.iCloud does it's daily backup at 12am providing the phone is plugged in, asleep and connected to WiFi.
That will be the culprit.
FWIW, Carrier can tally and count usage w/o contacting the phone.
Chances are, it isn't and that that's the time of the day when your carrier tallies and counts some of your day's usage.