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

download the appropriate firmware update and install it.

your mac will restart.

once restarted, goto

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