This post will show you how to enable power nap in osx mountain lion.

you need to have an SSD to enable this feature.

if you have a 2011 or 2012 macbook air, or the new retina macbook pro then go here http://support.apple.com/kb/HT5394

download the appropriate firmware update and install it.

your mac will restart.

once restarted, goto

system preferences > energy saver
tick the ‘enable powernap’ option.