Does Power Nap allow an Apple TV to access media on a sleeping Mac?
This seems to work on my AppleTV 2 and MacBook Air. 

Last night I turned on the AppleTV to browse my main library on a Mac Mini and the MacBook Air library showed up as well, even though the lid was closed and the computer was asleep. 

I have not experimented enough to know if this works with the power adapter unplugged. 

This was a very pleasant, but apparently undocumented, feature to discover. Perhaps it arrived in the most recent AppleTV update.