Windows 8.1 HTPC goes to sleep while Plex is streaming

While streaming to my Roku plex app my HTPC will go to sleep (flashing light on power button). I can wake it right back up via WOL but that's annoying. The media is stored on the same machine via three hard drives using drivepool. So the HTPC is not connecting to a NAS, all media is on the same machine. Is there a Windows 8.1 or plex setting i need to look at to keep the computer from going into sleep while a client is streaming? Thanks