Albert Barnes on Luke 9:32

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 are to bear in mind that this may have been in the night, and that they were weary with the toils of the day. Besides, they did not fall asleep while the transfiguration lasted. While Jesus was praying, or perhaps after he closed, 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, having been probably awakened by the shining of the light around them.

(x) "heavy with sleep" Dan 8:18, 10:9 (y) "they saw his glory" Jn 1:14