Mountain Lion Power Nap: Connected Standby
 

Mountain Lion now features connected standby via a feature Apple calls Power Nap. Even while sleeping your Mac will now fetch new emails, tweets, etc.. as well as download updates while it's plugged in. Power Nap is only supported on the 2nd generation MacBook Air or the new MacBook Pro with Retina Display.

http://www.anandtech.com/show/5981/m...nected-standby