After some Googling and reading some posts on this forum, it turns out that some people are having issues where their ZenFone spontaneously reboots at night during charging.
I'm making this topic to post my findings, as I was having the same issue. In the hopes that ASUS picks it up and can release a fix for it.
From my own tests, it seems that this only happens for me when the phone is charging overnight (wired, or wireless, doesn't matter) and either the WiFi or Mobile signal is weak. Last week I was abroad on a location with very poor mobile signal. There, my phone rebooted pretty much every night. Back at home, the issue resolved itself. Until I made some changed to my home WiFi and I had very poor reception in the bedroom for a few days. During this time, I saw my phone constantly reconnecting to the WiFi and it would reboot some time during the night when charging. When the WiFi issues were resolved, and I had good signal again, the issue went away and my phone now no longer spontaneously reboots.
Your mileage may vary, but for me at least it seems that the issue only occurs during this particular scenario. Hopefully anyone else or ASUS can reproduce and come up with a fix for the issue?