Why do I get billed for data usage in the middle of the night if i am hooked up to wifi?

It says in your online usage reports, data is delayed and often reported in 3 hour batches.  

Data is not reported live, it is delayed 3 to 72 hours domestically and can be delayed weeks if its roaming data.

Data use is calculated and reported once we stop using an app, system, when we connect to wifi or turn our phones off.  

Here's that reporing info from the on-line account @zwitch4me

From logging into my att.com account.

Even if data reporting were live (which it isn't), phones could very easily use cellular data when at home over night with WIFI available.  Most smartphoens will use teh best network connection available, which means that if you have slow DSL service and fast LTE cellular service, the cellular would be the better connection and would likely be used.  Also many phones are setup to turn the wifi signal off when the screen goes to sleep as a way to extend the battery life a few minutes, but at the same time, it will case the phone to use more cellular data when you think it should be using wifi.  That is a gimmick that the phone makers play to claim that their phone has the longest battery life, or longer than previous generation phones. 

I like to think of cellular data like a checking account.  The bank that holds the account is the final authority on how much money I have in the account.  WHen I write a check, I log it into the check register in my checkbook, and some time later, the bank actually processes it and removes the funds from my account.  At the end of the month, not every check I have written during the month has been processed by my bank, and some checks I wrote the proevious month have finally been processed by my bank.  If my bank has and fees, they are applied to my account and often come off the balance before my outstnading checks are processed.  When it comes to cellular data, think of AT&T as the bank, and your phone as the checkbook.  WHen you use cellular data, your phone records it immediately, but AT&T only records it against your account when it processes through the many systems to get to the billing system.  That often happens in the middle of the night, just like bank check processing.  At the end of the month, some of your data usage may not be processed in time to make it on this month's bill, while some stuff that missed last month's bill will appear on this month's.  You just need to keep a full register of your usage to "balance" your account with the "bank".  I know many "kids" today won't knbow what I am talking about since they never had to balanbce a checking account statement, but it is still the perfect analogy.

I have this same problem.  I spoke to an AT&T person who knew nothing, said he was transferring me to Apple, but sent me to an another clueless AT&T person.  The second person sent me to Apple, and I am now on hold with awful music for over 40 minutes.

I never used more than 5G, suddenly 22G and AT&T throttled my speed.  AT&T seems not to care 'you have unlimited data.'  

Sometimes it is the same as the other person, 3am and 6am.  Sometimes is is all night, every night at random intervals.  Literally an additional 17G???  And no one can tell me why.  Except maybe Apple, but no reaching them.  Since I use my iPhone at night in case my college kids call (they never do), I can't turn it off.  Guess I could Airplane mode it, but WHY?  Someone should be able to tell me what is up.  BTW, all my 'automatic' updates or anything are off, have always been off.

Your phone logs which apps are using data, so that’s the easiest way for you to see how much data you are using.