What is the significance of using "إذ" (when/as) with the night in "والليل إذ أدبر" (and the night as it recedes) and "إذا" (when/if) with the morning in "والصبح إذا أسفر" (and the morning when it appears) in Surat Al-Muddaththir?

Readers differ in their recitation of (إذ - idh) and (إذا - idhā) in the two instances. Some recite "إذ أدبر - idh adbara" (when it departed), and others "إذا أدبر - idhā adbara" (when it departs), and both readings are correct in meaning. Both "إذ" and "إذا" are temporal nouns that function as ḥāl (state) for the night and the dawn. Variation in speech is a custom of the Arabs. So, "إذا" is a temporal adverb for the future, and "دبر - dabara" means "أدبر - adbara" (departed).