Once you're on your Home Screen, scroll up to the toolbar and scroll to the right until you reach Settings.
Enter your Settings.
Once in your Settings, scroll down until you find the option Devices.
Enter Devices and select Controllers.
Under the Controllers menu, scroll down and select Brightness of DUALSHOCK 4 Light Bar.
From here, you will have three light options: Bright (Standard), Medium, and Dim.
As you can probably guess, Bright (Standard) is the factory setting for your DUALSHOCK and the brightest option. While you can't turn off the Light Bar altogether, Dim is the darkest option available, and it should be suitable for gaming in the dark.
If you need a little extra light, you can always make it brighter because you're now an expert at dimming your PS4 DUALSHOCK Controller! You can also check out our guide on how to extend the life of your PS4!