Al-Qasas 28:71
The verse stages a thought experiment in two symmetrical sentences: what if night never ended? What if day never ended? The word "sarmad" means unbroken, lasting without end.
قُلْ أَرَءَيْتُمْ إِن جَعَلَ ٱللَّهُ عَلَيْكُمُ ٱلَّيْلَ سَرْمَدًا إِلَىٰ يَوْمِ ٱلْقِيَـٰمَةِ مَنْ إِلَـٰهٌ غَيْرُ ٱللَّهِ يَأْتِيكُم بِضِيَآءٍ ۖ أَفَلَا تَسْمَعُونَ
Say, “Have you considered? Had God made the night perpetual over you until the Day of Resurrection, which god other than God will bring you illumination? Do you not hear?”
Al-Qasas 28:71 · Translation: talal-itani
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Either scenario costs its inhabitant something: unbroken night deprives of sight and light, unbroken day of stillness and rest. The Qur'an frames creation here as a balance: the lengthening of one is the diminishing of the other, and so the cycle itself is the gift.
Observation
Modern biology shows how thoroughly living things depend on the circadian rhythm. Animals kept under constant light suffer disrupted sleep and metabolic strain. Plants in continuous darkness cannot photosynthesize. At planetary scale: a world that did not rotate on its axis would have one face baked to hundreds of degrees before its star while the other froze — astrobiology calls this tidal lock. The verse's "unbroken night / unbroken day" thought experiment can be set beside this planetary picture; the verse offers no model, only the insight that the cycle itself is the gift.
Exegesis
Classical exegetes draw attention to the structural symmetry of these two verses: the same question is posed twice in two opposing scenarios; the symmetry carries both the logical and the rhetorical force. In the tradition of contemplation this verse is remembered as "the measure of gratitude": it brings the value of a blessing you already possess by showing what its absence would look like. To wake in the morning and to rest in the evening — without either, would you even have noticed?
Sources
Tafsīr: Abū ʿAbdillāh al-Qurṭubī, al-Jāmiʿ li-Aḥkām al-Qur'ān; classical tafsīr tradition. Science: planetary thermoregulation; tidal locking (habitable-zone discussions in astrobiology).