Dim the screen or use Auto-Brightness to extend battery life. Use Night Shift to shift the colors in your display to the warmer end of the spectrum at night and make viewing the screen easier on your eyes.
Adjust the screen brightness. Go to Settings > Display & Brightness, then drag the slider. If Auto-Brightness is on, iPhone adjusts the screen brightness for current light conditions using the built-in ambient light sensor. You can also adjust the brightness in Control Center.
Use Night Shift. Go to Settings > Display & Brightness > Night Shift. Night Shift shifts the colors in your display to the warmer end of the spectrum, making viewing the screen easier on your eyes when it’s dark.
Turn Night Shift on manually, helpful when you’re in a darkened room during the day, for example, or schedule it to turn on automatically. If you schedule Night Shift to turn on from sunset to sunrise, iPhone uses the data from your clock and your geolocation to determine when it’s nighttime for you.
To adjust the color balance for Night Shift, go to Settings > Display & Brightness > Night Shift, then drag the slider toward the warmer or cooler end of the spectrum.
You can also turn on Night Shift in Control Center. Tap 
.