Gene L. Green in The Letters to the Thessalonians (PNTC) has this helpful comment and reminder about Paul's words in these two verses:
The believers' existence as "children of the light and children of the day" has moral implications that the author begins to elaborate in [v. 6]. The imperative is put in the first person plural and begins with the words so then, which introduce the inference drawn from the previous statement (v. 5). Since Christians are "children of the light and children of the day," they should not sleep but rather be alert and self-controlled. This intimate relationship between their new existence and their new moral life touches a fundamental aspect of Christian ethics: What they are is what they should do. The moral exhortation finds its roots in the previous work of God in their lives. They have been made "children of the light and children of the day" via their salvation, and now they are to act according to that new state of being. The gift of grace includes within it the call to obedience. As V.P. Furnish states, "God's claim is regarded by the apostles as a constitutive part of God's gift. The Pauline concept of grace is inclusive of the Pauline concept of obedience." Since the imperative is integral to the indicative, the summons of Christian ethics becomes, "Act what you are." (pp.237-238)
Breaking off the indicative and emphasizing the imperative leads to gospel-less moralism and even legalism.
Breaking off the imperative and emphasizing only the indicative leads license and cheap grace.
What God has joined together (grace and obedience) let not the Christian or the preacher separate.
For the grace of God that brings salvation has appeared to all men. It teaches us to say "No" to ungodliness and worldly passions, and to live self-controlled, upright and godly lives in this present age, while we wait for the blessed hope--the glorious appearing of our great God and Savior, Jesus Christ, who gave himself for us to redeem us from all wickedness and to purify for himself a people that are his very own, eager to do what is good. (Titus 2:11-14)