Before you ask, I deny most cookies except essential ones, and I do not click on "keep me logged-in", for security reasons.
I would suggest that "there's your problem", as they say.
I've checked your account and there is nothing non-standard about it, so the problem is almost certainly at your end. There's no process that logs you out while logged in for server load, but 1 hour of inactivity (3600 seconds) will terminate the login session. Note that typing into the editor doesn't count as "activity", until you interact with the server again by pressing "Submit". If you have "keep me logged-in" selected, then the re-login is transparent and it appears that you have never logged out (or been logged out).