Dhun-Nurain (II)

It has also been mentioned in a narration that he was called Dhun-Nurain due to the fact that he recited a large portion of the Holy Quran during the Tahajud prayer [predawn voluntary prayer] every night. Since the Holy Quran is light and so is waking in the night to offer prayers, he became known as Dhun-Nurain, or the possessor of two lights.

(Ali Muhammad al-Salabi, Sirat Amir al-Momineen Uthman (ra) bin Affan Shakhsiyyatuhu wa Asruh, Ch. 1 [Beirut, Lebanon: Dar al-Ma‘rifah, 2006] p. 16)