5 You are all sons of light and sons of the day. We are not of the night nor of darkness.
6 Therefore let us not sleep, as others do, but let us watch and be sober.
7 For those who sleep, sleep at night, and those who get drunk are drunk at night.
8 But let us who are of the day be sober, putting on the breastplate of faith and love, and as a helmet the hope of salvation.
Paul, in these verses, explains why the Day of the Lord won’t over take believers like a thief.
all (v.5) — emphasized in the Greek — what Paul is about to write is true of all believers
sons (v.5) — describes the relationship between believers and God in virtue of their position in Him and emphasizes the dignity of that position
sons of the day (v.5) — continuing on the theme of light and not a reference to the Day of the Lord
we (v.5) — for a similar transition from “ye” to “we” see 1 Thessalonians 4:4 and 7. The underlying thought seems to be: “that which characterizes you is common to all believers, we are not of night nor of darkness.” Through the remaining verses of this section the writer uses the first personal pronoun. — Vine, page 76.
of the night (v.5) = belonging to the night
sleep (v.5) = katheudo, a different word than koimaomai, which is translated “sleep” (death) in 1 Thessalonians 4:13 — Katheudo can mean physical sleep, but here, it is moral sleep, spiritual sleep, indifference to spiritual things (Ephesians 5:14)
let us watch (v.6) = mental alertness, vigilance against a foe, the opposite of sleep
sober (v.6) = lit. “freedom from intoxicants” but used in Scripture metaphorically for “freedom from credulity and excitability — stability, the opposite of drunkenness
those who sleep, sleep at night (v.7) — In this statement of the ordinary fact of human experience the metaphorical language of the preceding verses is explained. As sleep is natural in the night, so indifference to God characterizes man in his unregenerate state. But for regenerate man to be spiritually asleep is to seem to be of the night, not of the day, of the world, not of Christ. — Vine, page 78.
we (v.8) — emphasized in the Greek for contrast
putting on (v.8) = clothing one’s self
The difference in the tenses of the verbs is suggestive, for whereas that of “be sober” is continuous, that of “putting on” is momentary. Believers are to continue in sobriety, but to exhort them to continue putting armor on would be to assume that on occasion they might lay it aside; this the Christian may never do. Hence the “point” tense, signifying “put your armor on, and keep it on.” — Vine, page 78.
breastplate (v.8) = a piece of armor which protects the body between the neck and the waist — Ephesians 6:14-17
hope (v.8) = expectation, trust, confidence
Until the Lord comes the believer will be surrounded by the enervating influences of a world bent on ease and pleasure, hence his eye is directed to the future in order to encourage him to resist the spirit of the age in which he lives. At Ephesians 6:17 “hope” is omitted, since there the believer is represented as engaged in conflict, and salvation as a present experience. — Vine, pages 78-79
Paul is not merely saying that “we” are of the light while “they” are of the darkness, but that since we are “the [sons] of the day” and “not of the night” (v.5), we belong to another category entirely. We will have been “caught up” and “the day of salvation” will have been brought to a close when the thief comes “in the night” to plunder this Christ-rejecting world.
And it is on this basis that we are exhorted not to sleep but to watch and be sober (v.6). We are not instructed to be awake and ready to repel the thief, for the day of the Lord cannot overtake us as a thief, for the simple reason that the day of the Lord will come (as a thief in the night) after we have been caught away (1 Thessalonians 5:4-5). Thus the apostle exhorts us to “watch and be sober” for the best of all reasons: because the dark night of the Tribulation has not yet come and for us it is still day. “… we are not of the night, nor of darkness. THEREFORE let us not sleep as do others; but let us watch and be sober” (vs. 5-6).
According to v.7, nighttime is the time when people sleep or indulge in revelry, but for us it is not night, thus “… let us, who are of the day, be sober, putting on the breastplate of faith and love; and for an helmet, the hope of salvation” (v.8).
What is this “hope of salvation” of which he speaks in v.8? Is it our salvation from the penalty of sin? Not exactly; it is our salvation from “the wrath to come” and indeed, our complete salvation (cf. Romans 8:23; 13:11), for again he refers to the coming of the Lord for us (vs. 9-10). — Stam, pages 94-95.