Night Shift was first introduced by Apple on March of 2016 with the release of iOS 9.3.  The feature effectively reduces blue light output by changing the display to a warmer yellow color.  It can be enabled either manually or set to automatically turn on and off based on your location.  Unlike many Android apps, Night Shift performs perfectly given the high level of integration in iOS.  Credit to Apple on taking the initiative to do this.

Night Shift settings may be found in Settings under Display & Brightness.  We very much appreciate that Apple has included a Color Temperature slider in the Night Shift settings allowing users to determine how aggressive they wish the reduction in blue light to be.

Night Shift setting requires a 64-bit processor found only in devices that use the Apple A7 or newer CPUs.  This means that older iOS devices such as the iPhone 4S, iPhone 5, iPhone 5C, iPad to fourth generation iPad and 5th Generation iPod Touch are all incompatible with Night Shift.