This is an issue that's actually been happening for me since the 5S. But notice more, and happens more frequently since getting the 6S. Intermittently, overnight, my phone would reboot on it's own. I know this because whenever I go into my phone for the first time the next day, I have to re-enter my code manually (I have finger print recognition on). I have seen my phone reboot on it's own during the day, but that's only happened twice with this new phone. The rest have been overnight. At least 5 times since upgrading to the 6S. Can't recall how many times on my 5S. I finally came across a tip in my search to what may be causing this. The tip suggests I go to Settings -> Privacy -> Diagnostics & Usage -> Diagnostic & Usage Data to see activities in or around the time I suspected it to reboot on it's own. And to look for something that pops up consistently. Turns out, what I see 8 in a row (3 of which has the dates I recall my phone rebooting on it's own overnight), is CoreTime-2016-xx-xx-xxxxxx.ips.synced (eg. CoreTime-2016-03-30-200234.ips.synced. This one happened last night). What is the "CoreTime ips synced" thing? What is it's purpose? Is it required? How do I prevent it from coming up/rebooting my phone? I'm absolutely certain that updating my iOS will not fix this issue. Why? Because I've updated my phone twice already, and reset it twice since purchasing it in January, and since day one of purchasing it, the random reboots were already happening. I'm thinking it's not the phone, but the iOS. Like I said, this has been happening since my 5S running iOS 8. This CoreTime is the most obvious suspect. Another potential one, but doesn't show up as much, is JetsamEven-2016-xx-xx-xxxxxx.ips.