Those large quantities of night-time data transfer that appear on your AT&T mobile phone bill? It's not new, and it's nothing to worry about - even though somehow it's getting more attention now. It's just routine updates for billing.
As you can see in this screenshot, the night transfers are as big as 75MB:
And AT&T's official statement is:
This is a routine update of your daily data activity on your device to ensure the accuracy of your data billing. Customers are not charged for data usage, given that no data session is generated. It's not uncommon for devices that are ‘always on', like iPhone, to process data event records for billing purposes after a certain amount of inactivity or after long periods of time. It's also separate from how our system lets you monitor your data consumption.
So even while you may have transfers listed during the day and then these transfers at night, your iPhone is not really uploading data to the internet at night. Good. Nothing to see here, folks, drive through. [Apple Discussions]
Updated by Jason Chen.