Why does my Asus PB277Q screen keep waking up while the Mac it is connected to is sleeping? Whenever I put my Macbook Pro running High Sierra to sleep my Asus PB277Q keeps waking up displaying a blue screen saying 'HDMI no signal' every few seconds, forever. Some other times it sleeps normally, and I can't tell what is causing the behavior. 

From what I can tell the Mac itself is asleep. It makes no sound, Bluetooth is off, Wake for network access is off, sharing (internet for example) is off, removing USB devices changes nothing. The screen is connected to a Displayport to HDMI/DVI/VGA adapter, which in turn is connected to the screen over an HDMI cable. 

Any ideas what else I can try?
This is because this monitor is for 75 hz but in high sierra or later I think this feature not work good now, I have this monitor and I have this problem, but you can use this monitor with your make, you go to display settings, select this monitor and change HZ 75 to 60 and it's works good.