Update - have changed motherboard to one with a Win10 compatible chipset and the problem has disappeared. The computer does not go into sleep mode, does not wake up from sleep mode, or wakes up from sleep mode unexpectedly. In summary if something in your PC/laptop is not compatible with the latest win10 update you can spend a lot of time chasing your tail until you hit a brick wall. The PC still works well when running but I am going to change the mobo for one that supports win10. It's not obvious until I contacted asrock who confirmed this.
I don't like to be beaten and have spent a lot of time going around in circles, the PC was a high end spec when built in 2011 but the root of the problems are due to the graphics chipset on the mobo do not support win 10. In the ASUS Monitor settings I've unchecked DisplayPort Deep sleep (turned it off) Turned off Windows Fast Startup Have my Windows 10 power settings set to High performance Updated all drivers and windows updates I'm using the displayport cable that came with the monitor but I've ordered a new cable and will try that. The Windows 10 try to save energy and unbidden, but the NVidia adapter can not be shutted down, due to the adaptive mode.
Go to Display setting > Other options, and disable the deep sleep mode. Now the PC is in hibernation mode the 2nd monitor will not wake up until I restart the PC. Press the buttons on the monitor to bring the setting. I searched BIOS for about 30 minutes in hope that I will find USB settings for waking up by keyboard or mouse, nothing. This all happened after I updated my BIOS to newest (4207) version, my motherboard is Asus A320M-K.
Note: Problems arose after switching from Windows 8.1 to 10. When in sleep, the mouse, keyboard, and power button all successfully wake the fans and PowerOn light, but not the monitor. I looked around and this seemed like an issue with older NUCs that has since been resolved. Display wont wake up by keyboard or mouse when monitor goes to sleep. The monitor does not receive input when attempting to wake from sleep. When the NUC is connected with an HDMI monitor (Samsung 4k monitor), the computer will intermittently not wake from sleep. My latest issue has been related to sleep mode start up, without going into too much detail this was overcome by changing from sleep to hibernation. I recently purchased an intel NUC 10 with a Core i5 processor ( NUC10i5FNK1). I have increasingly had issues with the graphics side since upgrading to win 10, most I have lived with or overcome to a certain extent. I have a PC with dual monitors and use the latest version of win10.