I loved that Jailbreak app where you could dim it further thanks to an overlay. (forgot the name) They should really include this standard in iOs.
Actually, there is a way to dim your iPhone 6's display at night, it's a hidden feature in the settings. Go to
Settings -> General -> Accessibility -> Zoom -> Enable Zoom
Tap the screen quickly with three fingers three times
Press ‘Choose Filter’ > ‘Low Light’, then set Zoom Region to “Full Screen Zoom”
Settings -> General -> Accessibility -> Accessibility Shortcut (it’s at the bottom) -> Set to “Zoom”
Tap the screen quickly with 3 fingers two times to zoom out
Now, simply press the home button on your iPhone or iPad three times quickly and the screen will dim without zooming in. Press it another three times and the brightness will return to its previous settings.
iPhone 6's screen is too bright???
You haven't used a Samsung @ Night with their Super AMOLED screens, give it a try and you'll see even at the darkest settings it will be beyond too bright.
Actually, there is a way to dim your iPhone 6's display at night, it's a hidden feature in the settings. Go to
Settings -> General -> Accessibility -> Zoom -> Enable Zoom
Tap the screen quickly with three fingers three times
Press Choose Filter > Low Light, then set Zoom Region to Full Screen Zoom
Settings -> General -> Accessibility -> Accessibility Shortcut (its at the bottom) -> Set to Zoom
Tap the screen quickly with 3 fingers two times to zoom out
Now, simply press the home button on your iPhone or iPad three times quickly and the screen will dim without zooming in. Press it another three times and the brightness will return to its previous settings.
Inverted colors is your only option. I occasionally use that when browsing at night.
EDIT: here's an iMore article about "dimming your screen more than control center allows."
http://www.imore.com/dim-your-screen-further-control-center-allows-and-other-cool-ios-8-tricks