With certain exceptions, the night is dark. If you use a light bulb will have, or if we see a clear sky at night but as day.
The night is dark and day light such as black and white, good and evil, these contrasts there seems to be one in harmony with each other, a dualism. We must not believe that dualism is something eternal, without end or beginning, no. This duality applies mostly 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 changes according to event perception. If we like some time with something that will pass quickly, but if a tooth hurts us terribly time will pass very difficult. However physical time was identical.
Certainly, if our world was eternal day and night did not exist, we would not appreciate the beauty of the day light. And certainly if our world would not exist River we did not understand what good is good.