Mac OS X: Why your Mac might not sleep or stay in sleep mode
Energy Saver settings: If the computer and display sleep timers are set to "Never", the computer will not sleep.
I have a Mac Pro running 10.7.5 My settings are all set to never and my machine goes to sleep. The screen freezes with a spinning wheel and I see the screen saver, the machine is not frozen as the server is still running but I have to force restart it to access the desktop. Any idea why?