When one sleeps, it actually appears like the person is a corpse. (When I sleep, I snore, but that is different.) When my children were very, very young and sleeping in the crib, I would actually nudge them to ensure they were not dead, because in sleep the resemblance is almost exactly akin to death (except for those who snore like me).
When the Bible talks about those who sleep (and who are still physically alive), the connotation is that their righteousness, which is received through faith, is not visibly evident, and therefore the person appears to be no different than someone who is unrighteous. That is, the believer and unbeliever are sometimes indistinguishable, because the believer is living a lifestyle that is no different than an unbeliever.
Thus the believer is “sleeping.” While they have righteousness, and therefore are “alive” to God, they are indistinguishable from someone who is a corpse (that is, someone who is an unbeliever, and therefore unrighteous). The living believer actually looks dead from a spiritual perspective, but since they are actually alive to God, they are called “sleeping” Christians.
We have to remember that no one is physically unconscious (that is, no one is literally sleeping in this imagery). Thus the “sleeping” Christian is someone who consciously lives in darkness, where the light does not shine. Their Christian testimony therefore is compromised.
What I have described in the previous four paragraphs was the problem with the Church in Sardis, whose Christians were “sleeping” (the walking dead). Their garments were soiled (Revelation 3:1-6), that is, their Christian testimony was compromised with the world, because they were sleep-walking in the darkness. They were NOT physically unconscious in any sense of literal sleeping, but they were indistinguishable from the unbeliever, who is dead (spiritual corpse).
Now the passage is Ephesians 5:14 is cut from the same cloth (same imagery, same concept, same problem, same idea). From what passage is Paul quoting in the Hebrew Bible? If we follow the logic that is described directly above, then the passage in question is Isaiah 60:1-2.
This passage in Isaiah 60:1-2 is NOT talking about physical unconsciousness, but in the original Hebrew it LITERALLY starts and says “Rise and Shine!” from the “darkness” from among the nations (unbelievers) so that the glory of the Lord can shine upon you.
The Apostle Paul therefore compressed Isaiah 60:1-2 into one verse in Ephesians 5:14, which (again) is NOT about PHYSICAL unconsciousness!!