Apple is always praising new iPhone displays as the brightest ever (having so many nits). This might be great if you are outdoors, but I tend to use my devices more at night or when it is dark and this is exactly when I don‘t want them to be as bright as possible.
So I turn down the display as much as I can.
This yields great results, but sometimes even this can be a problem. There is another option that you can choose, which is buried down in Settings in iOS 11.
For it to be displayed go to Display Accomodations (General > Accessibility) and Use Reduce White Point to further dim the display. This makes for a great and relaxing viewing which is easy on your eyes.