Apple has revealed a brand new feature of OS X Mountain Lion. They call it Power Nap.
Power Nap is essentially designed to let the operating system download updates, sync data, and do similar tasks even when the laptop itself is in sleep mode.
This works on charging and ensures that you are up to date when you turn the system on in the morning.
Intel has a similar technology that works on Windows powered laptops already.
When your Mac goes to sleep, it still gets things done with Power Nap.2 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.