Why did my Windows 10 restart?

If windows update restarts your machine, you should see an event from user32 stating the reason for shutdown:

It's followed by a bunch of logoff messages, and finally Kernel-General saying something like: "The operating system is shutting down at system time ‎2018‎-‎09‎-‎13T00:40:12.484519400Z.", followed shortly by a corresponding startup message.

If you don't see those types of logoff messages, you probably had a crash, and may find related error events somewhere in the event viewer.

If I'm not sure when a restart happened, I look at the EventLog events at 12pm every day that show the system uptime:

All of these logs are found under Event Viewer/Windows Logs/System.

There are several signs your PC has crashed, including the message on how "The last shutdown's success status was false." Because Windows doesn't have additional information in the crash, it probably wasn't a BSOD.

Unfortunately, further diagnosis is pretty much impossible because you haven't witnessed it. However, it's most likely related to how your "overnight computations" excessively stressed the PC. There may be shortcomings in the cooling/ventilation setup. Other than that it could be pretty much any hardware component being subtly broken.

To illustrate how a successful reboot (preceded by a regular boot) looks like, take a look at this event log. Note how there are messages of some services (including the event logging service) being stopped.