Apple has released a firmware update that brings the Mountain Lion Power Nap feature to Mid-2011 and newer MacBook Airs.

Power Nap keeps your Mac up to date even while it sleeps:

When your Mac goes to sleep, it still gets things done with Power Nap. 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.

The update called MacBook Air SMC Firmware Update 1.5 is available from Software Update.