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

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

To make it convenient at night to make the screen darker, you can turn on a shortcut by going to General > Accessibility > Accessibility Shortcut and click on Zoom. That way, everytime you triple-click the home button, the screen brightness lowers. To return it back to the normal state, just triple-click the home button again.