The pattern of waking during the day when it is light and sleeping at night when it is dark is a natural part of human life. Only recently have scientists begun to understand the alternating cycle of sleep and waking, and how it is related to daylight and darkness.
A key factor in how human sleep is regulated is exposure to light or to darkness. Exposure to light stimulates a nerve pathway from the retina in the eye to an area in the brain called the hypothala-mus. There, a special center called the supra-chiasmatic nucleus (SCN) initiates signals to other parts of the brain that control hormones, body temperature and other functions that play a role in making us feel sleepy or wide awake.
Taken from : http://www.sleepfoundation.org/article/sleep-topics/melatonin-and-sleep
"for, were they not aware that it is We who had made the night for them, so that they might rest therein, and the day, to make [them] see? In this, behold, there are messages indeed for people who will believe! "
~ An Naml 27:86
IT IS GOD who has made the night for you, so that you might rest therein, and the day, to make [you] see. Behold, God is indeed limitless in His bounty unto man - but most men are ungrateful.
~ Ghafir 40:61
In time We shall make them fully understand Our messages [through what they perceive] in the utmost horizons [of the universe] and within themselves, so that it will become clear unto them that this [revelation] is indeed the truth. [Still,] is it not enough [for them to know] that thy Sustainer is witness unto everything?
~ Fussilat 41:53
Translations of the Qur'an by Muhammad Asad