I might be wrong but I think that PowerNap works on iMacs with Mavericks and a standard HD I was away from my computer for 4h or so and when I came back my Time Machine backup was done when my mac was actually supposed to be in sleep. I also received all my Skype and mails notifications. I checked the PowerNap options in the System Prefs. pane. My mac and screen are configured to go in sleep after 15 minutes. Anyone else noticing this?