The idea of sleep is akin to appearing like you are dead: you are someone who lies motionless in the darkness. Thus in the Christian New Testament believers who live in the darkness of carnality appear like they are dead (unbelievers), but in actuality they are alive -- it's just that they appear as if they are dead.
1 Timothy 5:6 (NASB)
6 But she who gives herself to wanton pleasure is dead even while she lives.
Please bear in mind that throughout this imagery, no one is unconscious. That is, the sleep that appears as someone dead is not literal, but figurative.
These sleeping believers are also shameful, which is akin to taking your clothes off and exposing your nakedness. This shame is the humiliation of sinfulness.
Revelation 16:15 (NASB)
15 “Behold, I am coming like a thief. Blessed is the one who stays awake and keeps his clothes, so that he will not walk about naked and men will not see his shame.”
Thus the idea of sleep and taking your clothes off in the dark come together. In the following verse, clean white clothes represent righteous living.
Revelation 19:8 (NASB)
8 It was given to her to clothe herself in fine linen, bright and clean; for the fine linen is the righteous acts of the saints.
Earlier in the Revelation, the Church in Sardis faced the charge that some of them were sleeping. Again, the same imagery of someone who appears to be dead is compared to the person who is sleeping.
Revelation 3:1-6 (NASB)
1 “To the angel of the church in Sardis write: He who has the seven Spirits of God and the seven stars, says this: ‘I know your deeds, that you have a name that you are alive, but you are dead. 2 Wake up, and strengthen the things that remain, which were about to die; for I have not found your deeds completed in the sight of My God. 3 So remember what you have received and heard; and keep it, and repent. Therefore if you do not wake up, I will come like a thief, and you will not know at what hour I will come to you. 4 But you have a few people in Sardis who have not soiled their garments; and they will walk with Me in white, for they are worthy. 5 He who overcomes will thus be clothed in white garments; and I will not erase his name from the book of life, and I will confess his name before My Father and before His angels. 6 He who has an ear, let him hear what the Spirit says to the churches.’
In these verses, the believers do not remove their clothes to expose their shame (debauchery), but instead wear filthy clothes, which are works of self-righteousness. That is, these works have the appearance of righteousness, but are in fact "filthy wrong."
Revelation 22:11 (NASB)
11 Let the one who does wrong, still do wrong; and the one who is filthy, still be filthy; and let the one who is righteous, still practice righteousness; and the one who is holy, still keep himself holy.”
What is "wrong" includes self-righteousness, which is filthiness. The idea of the filthy garments of self-righteousness first occurs in Isaiah 64:6 and Zechariah 3:1-5, which the Book of Hebrews later echoes as "dead works" of self-righteousness -
Hebrews 6:1 (NASB)
1 Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God,
Hebrews 9:14 (NASB)
14 how much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, cleanse your conscience from dead works to serve the living God?
The believers in Sardis were sleeping, but instead of taking off their clothes and exposing their shame (debauchery), they wore their dirty clothes, which were the dead works of self-righteousness (pretentious piety). The Lord Jesus warned those who did not "shake off" this self-righteousness that their names would no longer appear in the Book of Life, which means that these believers were not sleeping (in the eyes of other believers), but were DEAD (unbelievers in the eyes of the Lord). Like the admonitions just noted in the Book of Hebrews, above, believers are to repent and cleanse their conscience of dead works, which are works of self-righteousness.
Finally, the Apostle Paul reminds us that we are from the light, and therefore we should be awake wearing our best clothes, which include the breastplate armor and helmet of God.
1 Thess 5:5-11 (NASB)
5 for you are all sons of light and sons of day. We are not of night nor of darkness; 6 so then let us not sleep as others do, but let us be alert and sober. 7 For those who sleep do their sleeping at night, and those who get drunk get drunk at night. 8 But since we are of the day, let us be sober, having put on the breastplate of faith and love, and as a helmet, the hope of salvation. 9 For God has not destined us for wrath, but for obtaining salvation through our Lord Jesus Christ, 10 who died for us, so that whether we are awake or asleep, we will live together with Him. 11 Therefore encourage one another and build up one another, just as you also are doing.
In the highlighted portion in verse 10, above, "the awake and asleep" refer to being alive physically ("awake") and to being physically dead ("asleep"), respectively. Like Stephen, who was stoned to death and "fell asleep" (Acts 7:60), the reference is to physical death. In the previous chapter (1 Thess 4:15-16), Paul had made reference to sleep to include physical death, but in the verses, above, Paul mixes both to refer both to "sleep" backsliding (verse 7) and to physical death "sleep" (verse 10).
In summary, all these passages and verses hail back to the Gospel accounts, when Jesus admonished His disciples to remain vigilant and awake for the Master's return.
Mark 13:33-36 (NASB)
33 “Take heed, keep on the alert; for you do not know when the appointed time will come. 34 It is like a man away on a journey, who upon leaving his house and putting his slaves in charge, assigning to each one his task, also commanded the doorkeeper to stay on the alert. 35 Therefore, be on the alert—for you do not know when the master of the house is coming, whether in the evening, at midnight, or when the rooster crows, or in the morning— 36 in case he should come suddenly and find you asleep.
The "appointed time" could be the hour of ones sudden death, but in this context refers to the Second Coming of Christ for His own. In respect of this imminent return, the Apostle Paul admonishes the Ephesians to WAKE UP, because they are not part of the darkness but of the light.
Ephesians 5:6-14 (NASB)
6 Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. 7 Therefore do not be partakers with them; 8 for you were formerly darkness, but now you are Light in the Lord; walk as children of Light 9 (for the fruit of the Light consists in all goodness and righteousness and truth), 10 trying to learn what is pleasing to the Lord. 11 Do not participate in the unfruitful deeds of darkness, but instead even expose them; 12 for it is disgraceful even to speak of the things which are done by them in secret. 13 But all things become visible when they are exposed by the light, for everything that becomes visible is light. 14 For this reason it says,
And arise from the dead,
And Christ will shine on you.”
In summary, believers are to remain awake because they walk in the light in the "clothing" of the armor of God (Eph 6:10-17). When believers enter into the darkness of carnality, they are asleep and therefore appear as dead people (unbelievers). In this state of carnal darkness, they either wear dirty clothes and thus produce dead works of self-righteousness, or they remove their clothes and expose their shame (debauchery). In the example noted in Sardis, some are believers were seen as asleep in the darkness of carnality, but in the omniscient eyes of the Lord, some were not asleep but were DEAD, which means that they were not believers, but unbelievers -- and thus their names will not appear in the Book of Life. The Apostle Paul leveled the same admonition to the carnal Corinthians. The Apostle Paul alluded to the Israelite "believers" who never made the journey to enter the Promised Land, and therefore were NOT believers (cf. Heb 3:16-19).
1 Corinthians 10:11-13 (NASB)
11 Now these things happened to them as an example, and they were written for our instruction, upon whom the ends of the ages have come. 12 Therefore let him who thinks he stands take heed that he does not fall. 13 No temptation has overtaken you but such as is common to man; and God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will provide the way of escape also, so that you will be able to endure it.
The issue is to take carnality seriously --which not only includes debauchery but also dead works of self-righteousness-- because those who are in carnal darkness may either (a) be sleeping believers, or worse, (b) be dead unbelievers... one rub in the passages of Revelation to Sardis and the passage by Paul to the Corinthians, above, is that sometimes the person concerned may believe in their own eyes that they fall into category (a), when in fact in the eyes of the Lord they fall into category (b).
In the eyes of fellow Christians, the difference between someone "asleep" and someone who is "dead" is sometimes indiscernible. In such cases, only the Lord knows those who are His, and therefore in this regard those who call on the name of the Lord should abstain from wickedness (cf. 2 Tim 2:19).