I know with the actual aluminum remote (for apple tv 3) that holding the play/pause button will do it, but doing that in the app doesn't seem to work for me.
I guess we need to ask why you want to put it to sleep? Frankly, it uses so little power that I actually set the sleep mode to off (so I don't even have to wait for it to come out of sleep when I want to use it). In our bedroom I threw a piece of electrical tape over the white light in the front (since it is tucked behind the television anyway).