Georgia, writing for iMore:

With iOS 9.3 Apple is introducing a new feature called Night Shift. As the name implies, it shifts the color spectrum of your iPhone or iPad display at night. So, why is that important?

And:

We do most of our healing when we sleep. It’s when our bodies repair themselves and our minds process the information we accumulated during the day. Lack of sleep can have an extremely adverse affect on our mental and physical health. It can interfere with our brain’s ability to form memories efficiently and even reduce our immunity to disease.

That’s where Night Shift comes in. When you enable Blue Light Reduction in the Display & Brightness settings, it moves the color spectrum from cooler (blue) towards warmer (yellow).

Really interesting.