After a time, my iMac goes to sleep - then tries logging me out

If I've been away for more than about half an hour, when I get back to my machine, I find that it has been trying to log me out. It has usually shut down the browsers and some applications and is paused with a message saying that terminal or excel prevented it logging out.

I don't want it to do this. I use the system as a server and I'm happy for it to sleep when not used, but not for it to shut down my session.

I thought it was because I'd got the power saving options set badly. However, I've got the schedule set to never startup or sleep (or shutdown or restart).

I'm wondering if this is part of the 'power nap' feature. Does that shut down applications?

Any suggestions?