Many people don’t realize the effect that light has on human, but the truth is that humans are enormously photo sensitive. It is light and dark that determines our Circadian rhythms, and sets our body clock.
The reason dates back to the time of manual typewriters. When it was first invented, they had keys arranged in an alphabetical order, until configuration was desired over convention.
UX; User Experience is often wrongly Interchanged with UI (User Interface), even by industries that offer employment to the latter. This misconception is one of the reasons mobile apps get low ratings in the app store.