One of the new features in OS X Mountain Lion is called Power Nap. Touted as a tool that get things done while your Mac is sleeping, Power Nap enables a Mac running Mountain Lion to download email, sync with iCloud, check for system updates, etc. while in sleep mode. There is a big caveat, however. Power Nap will only run on Macs with an internal solid-state hard drive (SSD).
During the developer beta phase of Mountain Lion and continuing into its public release stage, it was noticed that certain compatible Macs didn’t have Power Nap installed by default on the final release of Mountain Lion. Apple has since issued a firmware update to bring Power Nap to the MacBook Air, and now a new update today has been issued for the new MacBook Pro with Retina display.