Well, there are several issues to think of.
First when does the computer go to sleep, and how? Are you tracking idle time? Are you using acpid? pm-utils? cronjob?
What determines when the computer sleeps? Is there some measure of activity? Which activities are you tracking, and which do you ignore? Are you basing it solely on Tvheadend? Is it 30 mins of inactivity? 60?
What about other conditions to wake the machine? Setting the BIOS to wake at a certain time is fine for a recording, but what if you want to watch Live TV when the machine is asleep? Are you using magic packets (wake-on-lan)? If so, what it sending the WOL request? What about warmup/bootup time for the machine and tuning?