YSK: You can make your iPhone screen even darker than the minimum brightness by turning on Zoom with Low Light filter.

Go to Settings > General > Accessibility > Zoom > Zoom Filter and click on Low Light. Go back to the Zoom page and turn the slider on for Zoom. If your screen zooms in, double-tap on any part of the screen with 3 fingers to bring it back to normal view.

To make it convenient to make your screen darker at night, you should go to General > Accessibility > Accessibility Shortcut and click on Zoom. That way, every time you triple-click the home button, the screen brightness lowers. To go back to the normal state, just triple-click the home button again.