Mac reboots after some minutes of inactivity

I have this strange problem of my Mac rebooting. Everything was normal until I started syncing photos from iCloud (this could be coincidental, maybe not). First time I noticed a problem after 1-2 days as my system fan started going crazy, with CPU utilization at a max, several minutes later followed by system shut down. I traced the 100% CPU to Spotlight reindexing constantly; I temporarily disabled indexing:

sudo mdutil -a -i off

Problems with fans went away, but now photos service was going at very high CPU, accelerating fans again. I disabled Photos app (as much as I knew how) to avoid performing any analysis. This solved the fans and CPU going crazy problem. I could live without Photos on this Mac, no problems here.

Photos/Spotlight searching might just be coincidental, this part I'm not sure. Just so my system was more or less stable for at least several month before this addition.

But, here's the problem:

As soon as I leave my computer alone, after several minutes of inactivity, something happens that my computer goes into sudden reboot. I can't even leave it for 5 min for bathroom break. For information: putting computer to sleep was always disabled on my Mac, only screen saver and later on monitors were to trigger. Now I disabled it all, with no difference. I tried: 'caffeinate' with no difference as well.

Also, working on my Mac, performing various tasks has no effect whatsoever, but leaving it idle, goes to reboot soon.

Please help. What I was trying to find is if there's some kind of log that I could dump of a system on freeze, some entry to trace of what was the final activity before my Mac reboots. Any idea?

Solution 1:

Go to System Preferences | Security & Privacy | Click on "Advanced" on bottom right (unlock padlock first) | Uncheck "Logout after 10 minutes of inactivity"