My iPhone keeps using 4g data while I sleep even though it's connected to wifi, why?

[Christopher]

ok so I totally understand that I should get into the habit of turning cellular data off before I go to bed, but that's beside the point because it should NOT be using 4g when it's connected to the wifi and we have pretty good wifi in our house and as far as I know it doesn't get turned off overnight. Anyway I just checked my data usage for the day and it's over 100MB and I've been using wifi all day since I got up and my phone has been known to start with about 20 or 30MB used up (max, so far) when I wake up if I"ve left data on all night though for the most part it behaves completely with virtually 0MB over night... I'm not too too worried about it since I have the 3GB/mo plan and have yet to use more than 850 or so MB in a month (it's a bit harder than I thought it would be) but in about 30 days a month with about 3000 MB per 3GB, the average max per day is approximately 100MB/day to stay under so if this is going to start using 100MB per night, then it will become a serious problem. Anyway just wondering if anyone knows about this and how to fix it short of me remembering to turn data off overnight, which is no problem for me to do but I'd prefer not to because I use 4g a lot once I leave the house and would rather not keep switching it on and off when I may not have to, you know? it's an iPhone 4S, by the way, in case that info helps. (white is the color, lol)