Following on from the other tip about a quick fix to Lion Hanging issues, this tip shows you to get your machine to automatically reboot when it freezes.
If you are on a Macbook you need to go to System Preferences -> Energy Saver -> Battery Tab & Power Adaptor Tab, then click the checkbox for "Restart Computer if freezes"
For Desktop machines, System Preferences -> Power Tab the click the checkbox for "Restart Computer if freezes". With the desktop machine you can also schedule the machine to reboot at a certain time, i.e every sunday night etc.
Below are screen shots from the desktop and the macbook, with the macbook you can see the two different tabs.
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