iphone 4 using mobile data when connected via wifi

iPhone 4 using mobile data when connected via wifi
location: macrumors.com - date: May 3, 2012
My iPhone 4 is at the latest iOS version. Ever since I started using it in December I've found out that at the same time every day(midnight) when connected to wifi it will use a considerable amount of 3G. I only recently found this out last month when I went over my monthly allowance. I can only surmise that iCloud is somehow using the mobile data connection to backup when I've locked my phone, plugged it into the wall to charge and am asleep. Is there another rational explanation? My wife has the same phone and does the same thing as I do at night but her phone doesn't use any 3G data when backing up. Having checked my mobile bill for the last few months it is at exactly midnight that the phone starts using 3G data.....every night, without fail!! It's driving me crazy trying to figure out what's going on so any help gratefully received.
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