iPhone WiFi data usage / battery drain

I leave my iPhone overnight for maybe 10 hours, but when I get up in the morning the usage time usually goes up by 5 hours! Resulting in up to 25% battery drain.

This drain / usage only seems to happen when I'm at home within range of a WiFi network. At work I loose about 5% battery, and just minutes of usage.

No apps running (I have done a re-boot to be sure)
I have iCloud, yahoo and gmail accounts and I do not have push email enabled and fetch manual.
The location services for system items turned off.
I have no data in my calendar, so it cannot be generating reminders.
WiFi sync is not used, and my iMac is turned off at night.
Siri off, bluetooth off, photostream off, personal hotspot off.

When I check using the data usage app, my WiFi data counter has gone up several mb.

What can be causing this?

 

I have selected in iTunes to backup to my computer. Not iCloud.
Does it still do a backup? if so, surely it does not take 5 hours out of 10.