Does night mode on iPhone really help?
Different colors mean different wave lenght and energy, as displayed in the picture. 

Using Planck's theory, the shorter the wave is, the more energy it carries.
On day light, all these wave lenghts are present, including UV from sun. With no sun-light, your eyes aren't hit by those, so there's no need of using higher energy waves (that's exactly how night mode on iPhone works, using only lower energy waves).