Shortly before Apple introduced a new feature (seen in the latest betas of iOS 9.3) called Night Shift that reduces blue light emissions as the evening wears on to help fight eye fatigue and help with sleep, makers of a program that performs this function on Macs saw the iPhone maker reject a similar app for iOS. The OS X app is called f.lux, and was turned down because it used private APIs to manipulate the iPhone hardware, which is forbidden under the tighter rules for iOS.
