Never had any of our images going to sleep mode.
But if this is the pi 1 make sure that you have disabled starting x server/chromium. These cost lots of cpu power and I assume you have no display connected. This also controls monitor standby but don't think it would cause pi to sleep.
In recent images you just edit /boot/repetier-image-setup.txt and set RUN_CHROME to 0 then reboot.