I'm noticing a lot of little data usage during the night. every hour or so, it'll take 0.01mb from my plan. I'm connected to wifi. I tried checking wifi settings to see if it was going to sleep, but I couldn't find any settings like that in wifi.
iPhones like to eat data. When the screen's off, the phone usually prefers the data connection signal if WiFi is weaker. A simple solution is to just turn your data off and keep WiFi on when you're at home. That way no leakage occurs.