Due to the time it happens, I'm highly suspicious about WhatsApp and the Drive upload function.
I put my phone in Airplane mode before going to sleep, and this is what I see the day after.
Note that my WiFi appears as OFF when I wake up, yet the phone indicates it has been turned on for the whole night...
The Drive backup starts uploading in the morning when I turn WiFi on. But I think it's around 2 AM where you see my WiFi turning on (after WhatsApp backup?)