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?