I don't have a Pro, but my 1st gen Air seemed to have rebooted itself overnight aswell... I went to bed knowing that it was in landscape mode and Wi-Fi connected. I woke up and opened the smart cover to see any new notifications that would've arrived overnight, but when I opened the cover it was in portrait mode and Wi-Fi disconnected just like if it would've rebooted... Although it never told me that it rebooted and wanted my passcode.. My iPhone 6 was fine and I always leave them both charging overnight.
I had a look inside the log (Can be found at Privacy > Diagnostics and usage > Diagnostics and usage data) but I can't find anything related to a crash except for an .panic log from last year, but I don't recall it crashing at that time? It must've crashed/rebooted barely an hour after I went to bed since I got some mails within that time.