Even though Power Nap was one of the features Apple talked about post-release of their next-gen OS Mountain Lion, which was released two days ago now, it was missing from the release itself.

Apple today rolled out a new SMC update, which adds Power Nap to recent, compatible MacBooks. Power Nap enables a low-power state which provides just a very small amount of power to periodically download new e-mail, notifications, sync calendars, reminds, contacts and more.

If the MacBook is on AC power, the power state also allows the downloading and installing of security updates, backups, indexing and other battery-draining tasks, all while pretending its having a nap. During these tasks, a Power Nap-enabled MacBook will appear as though its just sitting there on standby, but between those updates and checks, the system will effectively be asleep.

Power Nap-compatible MacBooks only sit in an exclusive few, with mid-2011 and 2012 MacBook Airs qualifying, with the newly-introduced Retina MacBook Pro thankfully making the cut. One of the strict requirements of Power Nap is that the MacBook must be all-flash storage, which really limits to the Air range, and the new Retina MBP.