iPhones have a built in night shift mode that subtly makes your screen a little less blue, a little more yellow at night. But studies show that this is not completely effective as far as the circadian effects of blue light at night. Night shift mode makes your eyes feel a little less strained, but your melatonin levels are still suppressed. 
There is an accessibility feature, though, that allows you to shift the color spectrum of your phone completely towards the red end of the spectrum. Plus there is a way to set up a shortcut to easily toggle between normal and red mode.
1. Go to Settings -> General -> Accessibility
Click on Display Accommodations. Then Click on Color Filters.
Turn on Color Filters by sliding the on button. Then scroll down to increase the intensity. Red is the default color (almost like Apple knows that we want to eliminate blue light at night!)
Your phone should now be a lovely red hue – which is probably not something you want during the daytime!
Next you will set up a shortcut to toggle between the red mode and normal mode using a triple click of the home button.
Click back to go back into Settings -> General -> Accessibility
Scroll to the bottom and click on "Accessibility Shortcut".
Click on "Color Filters".
That is it! Give it a try by triple clicking the home button (three times, quickly).
I use this each night around sundown to shift my phone into red mode. This eliminates the melatonin-suppressing effects from the blue light that is normally emitted from the phone.
Added bonus - if someone calls or texts in the night and you need to turn on my phone, if it is in red mode it doesn't stop melatonin production and wake you up!