What Is The Power Nap Feature In OS X Mountain Lion?
While using Power Nap, your Mac may update its software. It also syncs Mail, Contacts, Calendar, and other iCloud components. Time Machine is also capable of backing up your Mac during Power Nap.
You can only use Power Nap if you have a Mac laptop with a built-in SSD. This currently only works with the MacBook Pro Retina Display and MacBook Air models from 2011 on.