Why does my iphone use data over night when cellular data is turned off?

Every night I turn off cellular data and leave the iPhone on and one day I wake up and check my data manager and see i have already used 25 MB please help!

There are various possible reasons:

1. Your phone (if you have iOS 5) backs up to iCloud automatically when it is asleep and plugged in.

2. Your phone can check for E-Mails periodically, and download them automatically for you to see.

3. Your phone can transmit usage reports to apple (does not compromise your pers info) unless you have the settings turned off for that.

4, Push notifications.

These are only a few reasons why your phone can do that. If you look in the Settings or the user manual there are settings you can change so that your phone doesn't do any of that. This is one thing to remember if you travel overseas with your phone. It could cost you THOUSANDS of dollars in data-transfer.