A recent firmware update pushed out by Apple enables the Power Nap in certain MacBook Air models that were promised the feature previously. This specific feature was found missing when OS X Mountain Lion launched earlier in the day.
First outlined in June during Apple’s Worldwide Developer Conference, Power Nap enables Macs to stay up to date and allows for automatic backup of data while the machine is sleeping. The automatic refreshing of data also includes Time Machine and iCloud syncing. The best part about the feature is that it is reportedly silent and power efficient although compatibility is limited to Macs that have solid state drives. Apple described Power Nap in the following way:
When your Mac goes to sleep, it still gets things done with Power Nap.3 It periodically updates Mail, Contacts, Calendar, Reminders, Notes, Photo Stream, Find My Mac, and Documents in the Cloud. When your Mac is connected to a power source, it downloads software updates and makes backups with Time Machine. While all that updating is going on, the system sounds are silent and no lights or fans come on, so nothing disturbs you. And when your Mac wakes up, it’s good to go.