For what the night is dark?

With certain exceptions, the night is dark. If we will use a light bulb, or if we see a clear sky at night but as day.
The night is dark and day light and black and white, good and bad, these contrasts there seems to be in harmony with one another, a dualism. You do not have to consider that this is something eternal dualism without end or beginning, no. This dualism is mainly applied our universe, three dimensional so.
As the night is dark and long, the day will be seen bright and brief. Exactly the opposite. But why?
Time perception change depending on the event. If we like some time with something that will pass quickly, but if a tooth hurts us terribly time will pass extremely hard. However the physical time was the same.
Certainly, if our world were eternal day and night did not exist, we would not appreciate the beauty of the day light. And certainly in our world if evil did not exist we would not understand what good is good.

Simple questions require simple answers. This site is meant to be a positive response to this need. answer-blog.blogspot.com