Why is there no Night Shift mode on my iPhone?

Update: The Night Shift toggle was removed from Control Center in iOS 11. Click here to learn how to manually enable it in iOS 11 or later.

Night Shift mode is a new feature found in iOS 9.3. You can enable NIght Shift mode in the Display & Brightness section of the Settings app on your iOS device.

Night Shift mode is only available on devices with 64-bit processors. This includes the iPhone 5s, SE, 6, 6 Plus, 6s, 6s Plus, iPad mini 2, 3, 4, iPad Air and Air 2, iPad Pro (both models) and iPod Touch 6G. If you own one of these models and Night Shift mode is not available in the Display & Brightness settings, navigate to Settings > General > About and check the Version panel to make sure iOS 9.3 is installed.

You can learn more about NIght Shift mode here.