Albert Barnes Commentary Luke 9:32

"Now Peter and they that were with him were heavy with sleep: but when they were fully awake, they saw his glory, and the two men that stood with him." — Luke 9:32 (ASV)

Heavy with sleep. Borne down with sleep—oppressed, overcome with sleep. It may seem remarkable that they should fall asleep on such an occasion; but we should bear in mind that this may have been during the night, and that they were weary from the day's labors.

Besides, they did not fall asleep while the transfiguration lasted. While Jesus was praying, or perhaps after he finished praying, they fell asleep. While they were sleeping his countenance was changed, and Moses and Elias appeared. The first that they saw of it was after they awoke, probably awakened by the shining of the light around them.