The light from LEDs is quite blue, so using it late at night or in bed can mess up your sleep because your eyes see the light and your body says "oh, it's day time! better get on with huntin' and gatherin'" I've tried an app on my android that dims the screen and makes things more orange, but that makes it hard to read. White text on a black background reduces the amount of blue light your eyes are exposed to, and also stops it disturbing any sleeping partners you may have**
** and as
@Markymark will shortly assure us, for most IOS users, that's more than one, on average