Albert Barnes Commentary Job 20:8

"He shall fly away as a dream, and shall not be found: Yea, he shall be chased away as a vision of the night." — Job 20:8 (ASV)

He shall fly away as a dream - As a dream wholly disappears or vanishes. This comparison of man with a dream is not uncommon, and is most impressive. See Psalm 73:20; see the notes at Isaiah 29:7-8.

As a vision of the night - As when one in a dream seems to see objects which vanish when he awakes. The parallelism requires us to understand this of what appears in a dream, and not of a spectre. In our dreams we seem to see objects, and when we awake they vanish.