OS X Mountain Lion: About Power Nap

Power Nap in OS X Mountain Lion lets some Mac computers stay up to date even while they’re sleeping.

To use Power Nap, your computer must use flash memory (not a hard disk or solid-state drive). When the computer goes to sleep, Power Nap activates periodically to update information. The information that’s updated depends on whether your computer is running on battery power or plugged into a power adaptor.

When your computer is asleep and using battery power, Power Nap:

When your computer is plugged into a power adaptor, Power Nap can also back up your computer using Time Machine.