Using a bright screen in a dark environment isn’t doing any favor to your eyes, and that’s why some apps incorporate a so-called night-mode for dark settings. But night mode features within apps have a big limitation, and that’s that they only work within specific apps.
Wouldn’t it be better if iOS had a true system-wide night mode that worked in apps as well as the iOS interface as a whole? That’s the basic idea behind Nightmode9, a recently released jailbreak tweak that adds tons of user-configurable night mode features to iOS 9.