FarFinder will not work if your Mac is asleep, because your Mac will not be able to receive network connections.
You can use the Energy Saver system preference to stop your Mac going to sleep.
If you don't want to do this, and your Mac is connected with a wired not a wireless connection, you might be able to use wake-on-LAN to wake up your Mac, then you'll be able to use FarFinder. You can read more about wake-on-LAN on Wikipedia. Some routers have this capability built in.