Reducing your iPhone's brightness level to the default at night may look too bright in some times, however, there's a way to turn down the brightness level below the default in your iPhone, iPad or iPod Touch without even using jailbreak..
That setting is hidden inside iOS’ wide variety of accessibility features.
Here's how to turn it below the default level:
Swipe up from the bottom of the screen, and turn the brightness all the way down from Control Center.
Open Settings and navigate to General > Accessibility > Zoom
Scroll down, and turn on the “Show Controller” setting.
Go back one page, and scroll down go to “Accessibility Shortcut”
Select Zoom. You can now access the Zoom menu by triple clicking your home button.
Go back to your home screen, and triple click the home button to bring up the Zoom menu. (If you don’t see the menu, tap on the circular controller to bring it up.)
Tap on “Choose Filter”, and select “Low Light”
You’ll see that the screen dims, and is more soothing to the eye. You can dismiss the zoom popover by tapping outside it. You can enable or disable this mode with the triple click shortcut you setup a few steps back.