Theophylact of Ohrid — as excerpted in the Catena Aurea on Luke 9:32-36

“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.”