Mac goes to sleep even when streaming to Apple TV?!

Hello everyone,

I just recently bought the Apple TV (3rd gen) to compliment my rMBP and am loving it.

I keep iTunes open with my Mac on, especially when I know I'm going to be streaming. However, my Mac seems to doze off half way through streaming a TV show, or a movie (usually something with a long play time). Macs have always just connected and "worked" for me so it seems strange that the ATV wouldn't just wake my Mac when I select Home Sharing.

If the show stops playing, sometimes if I go back and select Home Sharing again it will wake my Mac.. but often I get this message:

"Turn On Home Sharing in iTunes

To access your iTunes library on Apple TV, turn on Home Sharing in iTunes on your computer using <email>."

My Mac's energy savings are set to sleep after 1 hour, it's always plugged in. iTunes is always open. Do I have to set energy savings to 'always on' if I want it to serve the ATV whenever In want? I thought the ATV would be clever enough to wake the Mac when it wants something, which would save a lot of energy - and as I said it does work on occasion. As I have a connected USB hard drive I'm trying to do it this way instead of having an always-on NAS.

I should probably add that I'm using an AirPort Extreme with the correct firmware required to have the whole wake-over-wifi stuff.

Advice please :o)