The alternation of night and day is also counted among the signs. It is stated: “And similarly, your sleeping during the night and day and your seeking His bounty are among His signs. In this, indeed, are many signs for those who listen (with the ears of the heart).” The coming and going of night and day, outwardly, seems like a very ordinary event, yet within it, the extraordinary knowledge, wisdom, and nurturing aspect of the Lord are fully manifest. Allah knows that the beings He has created have a need for mental, spiritual, and physical rest. Without this, they cannot remain capable of functioning for long. Moreover, a constant and unchanging environment makes the activity of life monotonous. The requirement of creation in this manner is that the dwelling prepared for them should be in accordance with this style. That is, if the Creator is All-Knowing and Wise, He will be fully aware of the needs of His creation and will make arrangements for their fulfillment with perfect nurturing. This need and the means for its fulfillment are proof that humans and the universe did not come into existence by chance but were created by an extremely knowledgeable and wise being.
Javed Ahmed Ghamidi writes in explanation of this verse:
It was stated that if they would only reflect on how their Creator, with such mercy and compassion, has harmonized them and their environment by dividing the necessity of sleep and rest for the human body, and the struggle for livelihood into night and day. Does this not clearly indicate the existence of a Merciful and Generous Lord? Can a person still claim that all of this happened by chance, or that there could be the authority of more than one god imagined, or that the Creator of this universe would let it perish without purpose? In this, if you reflect, there is an implicit criticism on the behavior of the opponents: they hear, yet do not attempt to understand, and instead, they rise in opposition, blind and deaf. (Al-Bayan 4/54-55)
In Surah Yunus, this sign is described as a symbol of the end of the universe. It is stated:
اِنَّ فِي اخْتِلَافِ الَّيْلِ وَالنَّهَارِ وَمَا خَلَقَ اللّٰهُ فِي السَّمٰوٰتِ وَالْاَرْضِ لَاٰيٰتٍ لِّقَوْمٍ يَّتَّقُوْنَ.
Surely, in the alternation of day and night and whatever God has created in the heavens and the earth there are signs in it for those who fear. (10:6)
The meaning is that the cycle of days is a sign that the universe is reaching a great conclusion; it is not without purpose. Imam Amin Ahsan Islahi has stated:
The alternation of night and day also indicates the pursuit they are actively engaged in with each other, from which it is guided that this cycle is not aimless or without purpose, but is culminating in a great result. Secondly, it points towards the great system of Lordship that is inherent in the different temperaments of night and day, that day energizes the arena of activities for livelihood and economic endeavors for man, and night spreads a bed of comfort and rest for him. Whoever reflects on this system necessarily reaches the conclusion that this amazing concord within opposites for a common purpose can exist only in the form that it is accepted that this entire machinery operates under the will of one Omnipotent and Self-sustaining Being. It then concludes that He, who has established this entire system of providing and caring and is running it with such arrangement, will not leave man unaccountable and irresponsible. Instead, a day is bound to come in which He will reward those who recognize the right of this Lordship with the gift of their acknowledgment and will cast those indifferent to it into Hell. This is the conclusion achieved from reflecting on all the components of this universe and all its opposites, and it is this realization that guides humanity toward the hereafter and this recompense and punishment. (Tadabbur-e-Qur’an 4/26)