Night shift, or night mode is a setting that reduces the blue in your screen, turning everything a calmer, sepia-like tone. This post shows you how to enable night mode in ios11
swipe up from the bottom of the screen to enable the controls
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tap-and-hold on the brightness setting
the controls will change and show the night mode setting
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click on the icon at the bottom and night shift will be enabled
click again to turn night shift off