Early morning data spikes - hypothesis

Jan 16, 2008
Hypothesis: large data chunks that start downloading on cellular data, keep downloading on cellular, even if wifi subsequently comes online.

(It's a mystery to me and many others [according to the internet] why there are these late-night spikes. But I will accept that they exist.)

Symptom: I have large data spikes in middle of the night (if I forget to turn off phone) or first thing when I turn on the phone. (75% of my data use is between midnight at 6:40am, while I'm at home, where I have wifi.) These data use spike are being billed to my cell data plan, despite there being plenty of wifi in the house.

Sub-hypothesis: at phone startup, the cell network connects faster than wifi, big data download begins, wifi comes online, but big data continues on cell data network. (I expect something similar if wifi disconnects: downloads that begin in this state start on the cell network and stay there even after wifi reconnects.)

What does the community think of my hypothesis?

As a test, I plan to turn off cell data before nightly power-down, so that when I turn on phone the next day, data will be forced to wifi. I'll switch on cell data when I leave the house for work.

(PS: This may not be a new problem. I just started noticing because I've blown through my 200M data plan 2 months in a row: the first 2 months I've been back to work in 2 years. It's possible these data spikes were always there, but my under-200M/month usage did not draw my attention. Sadly, AT&T does not show past usage breakdown, only total, so I can investigate the time-of-day effect for past months.)
Jun 29, 2012
U could also make sure to disable saving your app data to iCloud via cell data...
Here's how you do this:

I have the same problem with my iPhone 4s. I've been contemplating switching to the shared plan Verizon offers, and it's worth it, as long as I stay below 2GB usage a month. However, the same spike (once up to 2GB in ONE NIGHT) happened overnight while I was home and connected to WiFi. Also, that's when my battery would drain completely in 4 hours of standby. So, something strange was going on.

I just changed the iCloud setting mentioned above. Hopefully, that makes the difference.