Gregory the Great — as excerpted in the Catena Aurea on Luke 1:34-35

“(18 Moral. c. 20. super Job 27:21.) By the term overshadowing, both natures of the Incarnate God are signified. For shadow is formed by light and matter. But the Lord by His Divine nature is light. Because then immaterial light was to be embodied in the Virgin’s womb, it is well said unto her, The power of the Highest shall overshadow thee, that is, the human body in thee shall receive an immaterial light of divinity. For this is said to Mary for the heavenly refreshing of her soul.”