iMac goes to sleep when rendering

I know this has been posted before, but all previous posts didn't have solutions.

When I render directly from Premiere, or AME, my iMac (27 inch late 2014 4GHz Quad-Core i7 with 32GB DDR3) goes to sleep as soon as the composition gets complicated (requires more abilities to render). Yes, all power settings are on maximum and prevent the computer from going to sleep, but the problem is recurrent. Once I press spacebar, the computer wakes up and AME continues rendering.

I have a 34 inch (3440 x 1440) extended display attached. I am suspecting that the Mercury Playback Engine GPU Acceleration (Metal) is putting pressure on my AMD Radeon R9, and forces the computer to sleep.

Would be grateful for any tips.