وَالنَّهارِ إِذا تَجَلّىٰ
By the day when it brightens.
EXEGESIS
Tajallā (brightened) is in the past tense from the infinitive tajallī which refers to the appearance of something after it had been hidden.
The verb envelops was used in the previous verse in the present tense whilst brightens in this verse has come in the past tense. Possible explanations for this difference were mentioned under 91:3-4.
EXPOSITION
This verse takes an oath by the day when it comes out of the darkness of the night, and this is also one of the great blessings of Allah, for if the earth would have constantly remained in darkness it would have been difficult for people to work, and similarly if the earth was always engulfed in daylight they would not be able to find the peace and comfort which comes at night.
The Quran frequently mentions the day and the night as one of the great blessings and signs of Allah (see 27:86 and 17:12, for example). The alternating of the day and night is also a blessing, for if one part of the globe were to remain in darkness permanently and the other side were to remain in sunlight likewise, life on earth would not be able to progress smoothly. Say: ‘Tell me, if Allah were to make the night perpetual over you until the Day of Resurrection, what god other than Allah could bring you light? Will you not then listen?’ Say: ‘Tell me, if Allah were to make the day perpetual over you until the Day of Resurrection, what god other than Allah could bring you night wherein you could rest? Will you not then perceive?’ Out of His mercy He has made for you the night and the day, that you may rest therein and that you may seek from His grace and so that you may give thanks (28:71-73).
Had the world remained in darkness, it would not have been possible for plants to grow as they need sunlight for photosynthesis. Animals would not be able to move around and birds would not be able to fly as they would not be able to visually examine their surroundings. Likewise, humans would also not be able to function efficiently and thus the Quran frequently mentions that the day is a blessing in which man is able to work and sustain himself (28:73). As for the night, it has been scientifically proven that our bodily systems are active and working during the day and if the day were to perpetually remain, this would damage the cells of our bodies as well as our other organs. The night gives our body a chance to rest and recover. Scientists call this the circadian rhythm, a type of internal clock that our bodies have and which is in sync with the day/night pattern, a combination of physical, mental, and behavioural changes that follow a 24-hour cycle. These natural processes respond primarily to light and dark and affect most living things, including animals, plants, and microbes.
[1] Mizan, 20/302.
[2] Tabrisi, 10/759.
[3] Amthal, 12/285.