I have my Mac Pro (Cyrus) configured not to fall asleep completely but to let the screen sleep when not used for half an hour. I also use the keyboard shortcut for forcing the display to sleep.
However, sometimes, for a few weeks the screen just refuses to sleep and the login alert box appears immediately after the sleep is supposed to start as if somebody violently moved the mouse or pressed a key. After a few weeks the behaviour changes again and sleeping works, for another few weeks. It's periodic.
How can I force the screen to sleep without waking up all the time?