I generally set up power hungry servers with a fairly short timeout before they're supposed to suspend or hibernate, but they seem to occasionally go to sleep during file transfers, ssh sessions, etc. What sort of activities are considered "activity" of the sort that will keep the machine awake?
SnapOverflow
As someone above has posted in a comment, it seems to be mouse or keyboard activity that keeps the machine awake. I'm not marking this answer correct, as it's not really mine, and I can't guarantee it's accurate at this point.