There is a tiny app called Caffeine. It is Free ware. And the little coffee cup icon stays in Taskbar & you click on it & your Mac won't go to sleep or run it's screen saver until you click it again. I use it for rendering in iMovie or burning DVD's. But if I remember correctly, your Mac going to sleep should not stop your TV from streaming. I'll have to test it but I'm almost sure of this.