Mountain Lion offers a new feature called Power Nap, but since it’s limited to only a few recent laptop Mac models, most people probably haven’t seen it. In short, Power Nap enables supported Macs to wake up briefly to back up via Time Machine, check for email via Mail, receive new messages in Messages, and update iCloud-related data, including calendar events, contacts, reminders, notes, iCloud documents, and photos in Photo Stream. Over at Macworld, our friend Dan Frakes explains just what Power Nap can do, how you use it, why it works on so few models, and more. follow link
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Dan Frakes Explains Power Nap
Something I have not seen discussed anywhere is a similar feature that already exists apart from PowerNap in Mac computers. My own 2010 MBA which does not support PowerNap performs a "Dark wake" each morning when I detach the power cord and then connects to my Airport network and downloads all e-mail, without me opening the lid of the computer. This is a real feature apart from PowerNap, but I am not sure in what models it works or how long it has worked like this. See the Console logs! -Jerry
It also seems to take consistently longer time to wake from sleep when opening the lid after it has previously made a dark wake than when it has not done so since you last closed the lid. I know this happened in 10.7 Lion too (but not sure it worked that well and maybe Mail messages were not downloaded). Mail messages download without using push. Not sure what other services works in Dark wake, but Dropbox does not sync. There were an article a year ago in Macnews on this: http://www.macnews.com/2011/08/09/apple-working-dark-wake-methods-macs