Mac MIni is going to login screen

[Yerman]

Every couple days, my Mac Mini Plex Server is going off-line (TeamViewer). When I get home to check, the Sleep Indicator Light pulsates on and off slowly indicating that my computer is sleeping. When I view the screen, my computer is at the login screen asking for my password which I thought I had disabled.

I am thinking I should just install Caffeine, but I'm concerned that my external HD is not going to sleep while running. Furthermore, I don't think I should have to run Caffeine....it should just not sleep based on my settings right?

Here are the screen shots of my settings. Any ideas?

 

[satcomer]

Power fluctuations have been known to cause this too. Is the server on an ups?

 

[Yerman]

Ya. I have it in my server/AV rack so it shares a large UPS. Good ventilation too.

Is there anything I can run to monitor what is going on