Desktop reboots itself in the middle of the night

Sometime between leaving my desktop at night and resuming its use in the morning, my computer restarts itself. I tend to leave my computer on unless I have some reason to restart it, because I run Folding@Home, and run backups at night.

Normally I notice this because I'll go to log in and all my programs are starting themselves and anything I had open isn't. I know it's not doing this every night, but I'm probably not aware of every time it does it because my grandfather gets up super early (generally between 4 and 6) and usually ends up playing solitaire (surprising, I know). He's not exactly tech-savvy but he's not likely to mention anything to me unless something is glaringly wrong (like the computer crashing while he's using it).

I haven't been able to find a generalized answer to my question that applies to me. The obvious answer is windows update. I have windows update set to require my explicit permission to download and install. I checked that the settings are still that way just this morning. I wasn't able to find any entry in the scheduled tasks that's an obvious culprit, but when a lot of them are just set to run an external program, it's hard to tell. Consider the following excerpts from this morning:

6:25:48, Information, Kernel-General: The operating system started at system time ‎2015‎-‎11‎-‎28T14:25:47.860398900Z.
6:26:13, Error, EventLog: The previous system shutdown at 6:24:09 AM on ‎11/‎28/‎2015 was unexpected.
6:26:01, Critical, Kernal-Power: The system has rebooted without cleanly shutting down first.

This seems to be a trend and obviously every time it's happened it's surrounded by various bits of information that could be the culprit or could be nothing, but I don't really know enough about it all to tell. I'll try to pay more attention to the system logs in the future.

Any ideas? It would make my life a lot easier if my computer wasn't tripping itself up in the mornings.