Luke 9:32

Or else, according to your capacity is the word either lessened or increased to you, and unless you ascend the summit of a higher wisdom, you behold not what glory there is in the word of God. Now the garments of the Word, are the discourses of the Scriptures, and certain clothings of the Divine mind; and as His raiment shone white, so in the eyes of your understanding, the sense of the divine words becomes clear. Hence after Moses, Elias; that is, the Law and the Prophets in the Word. For neither can the Law exist without the Word, nor the Prophet, unless he prophesied of the Son of God. For the incomprehensible brightness of the Divine nature oppresses our bodily senses. For if the sight of the body is unable to contain the sun's ray when opposite to the eyes which behold it, how can the corruption of our fleshly members endure the glory of God? And perhaps they were oppressed with sleep, that after their rest they might behold the sight of the resurrection. Therefore when they were awake they saw His glory. For no one, except he is watching, sees the glory of Christ. Peter was delighted, and as the allurements of this world enticed him not, was carried away by the glory of the resurrection. Hence it follows, And it came to pass as they departed, &c.

While Christ is engaged in prayer, Peter is heavy with sleep, for he was weak, and did what was natural to man; as it is said, But Peter and they that were with him were heavy with sleep. But when they awake, they behold His glory, and the two men with Him; as it follows, And when they were awake, they saw his glory, and the two men, that stood with him.