this post will show you how to enable powernap 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.