Had this problem.
Was updating drivers, getting new rst lan usb drivers etc. Decided to update bios to F4 at same time.
Took a day or two to notice that monitor sleep wasn't working anymore.
Spent hours upon hours upon hours troubleshooting. Commandline powercfg reports, disabling every service one by one and waiting, disconnecting all usb devices, pulling out sound card and disconnecting hard drives from mb. Rebooting to safe mode and doing it all again. Uninstalling any recent programs, uninstalling the drivers I'd gotten. Maybe it was corsairlink, maybe it was corsair ssd tool, maybe aida64, maybe my new kb, maybe my new mouse, maybe this maybe that.
Days of frustration.
Then I find this thread.
Try to revert to F3 bios. Didn't work, start procedures all over again. Notice that F3 bios from tweaktown is different from old F3 bios, different logo screen. Last try installed F5c bios from tweaktown. 2 minutes later monitor goes to sleep.
I'd just like to give a big FTHANKYOU to gigabyte for releasing a "stable" bios that breaks powersaving. It's obviously a known issue to you and yet there are no notes on the bios download page. Thanks for making me waste hours and hours of time trying to fix this. Thanks for all the frustration and rage.
Last gigabyte product I ever buy or recommend. Back to Asus for me.