The Sudden Darkness
macOS is highly optimized to preserve battery life and extend the longevity of its brilliant Retina displays. Consequently, if you step away from your Mac or stop interacting with the trackpad for a set period, the operating system will automatically turn off the display (putting it to sleep) while keeping the underlying system running. While excellent for conserving power, this is incredibly frustrating if you are actively reading a long reference document, monitoring a slow download, or presenting a static slide on a projector, as you have to constantly wiggle the mouse to keep the screen alive.
Keeping the Lights On
To ensure your screen remains continuously lit and stop macOS from automatically sleeping the display, you must adjust the energy and lock screen settings.
Click the Apple logo in the extreme top-left corner of your screen and select System Settings. In the left-hand navigation sidebar, scroll down and click on Lock Screen. In the main right-hand panel, look for the dropdown menu labelled Turn display off on battery when inactive (desktop users will only see a single “Turn display off” option, while laptop users will also see an “on power adapter” option). Click the dropdown menu, scroll all the way to the bottom of the time increments, and select Never. A warning prompt may appear reminding you that this uses more energy; click OK to confirm. Your display will now remain on until you manually close the lid or trigger a hot corner.