Luke indicated that Jesus said to be on guard or aware (Προσέχετε δὲ ἑαυτοῖς), so that their hearts should not be weighed down (μή ποτε βαρηθῶσιν ὑμῶν αἱ καρδίαι) with dissipation (ἐν κραιπάλῃ), drunkenness (καὶ μέθῃ), and the daily worries of this life (καὶ μερίμναις βιωτικαῖς). Luke was the only Greek biblical writer to use this term κραιπάλῃ, that means drunken nausea or drunken dissipation. Thus, the day of the end times would not suddenly catch you unexpectedly (καὶ ἐπιστῇ ἐφ’ ὑμᾶς αἰφνίδιος ἡ ἡμέρα ἐκείνη). There is something similar in Matthew, chapter 24:43 and Mark, chapter 13:35. Mark indicated that Jesus said that they were to be aware (Βλέπετε) and alert all the time (ἀγρυπνεῖτε), because they did not know (οὐκ οἴδατε) when the end times (γὰρ πότε ὁ καιρός ἐστιν) would come. Luke, chapter 12:39-40, also had something similar about the thief at night. Jesus warned his disciples to be vigilant. They were to stay awake (γρηγορεῖτε οὖν), because they did not know on what day (ὅτι οὐκ οἴδατε ποίᾳ ἡμέρᾳ) the Lord was coming (ὁ κύριος ὑμῶν ἔρχεται). Therefore, they had to be ready or prepared (διὰ τοῦτο καὶ ὑμεῖς γίνεσθε ἕτοιμοι) for the coming of the Son of Man (ὁ Υἱὸς τοῦ ἀνθρώπου ἔρχεται) because he would be coming at an unexpected hour (ὅτι ᾗ οὐ δοκεῖτε ὥρᾳ). This is also similar to the parable ending in Matthew, chapter 25:13, about the virgins at the wedding being vigilant. This was a simple message to be vigilant all the time, because your end or the end of the world could happen at any time. Are you ready to go?
Luke uniquely said that Peter (ὁ δὲ Πέτρος) and his 2 companions (καὶ οἱ σὺν αὐτῷ) were weighed down with sleep (ἦσαν βεβαρημένοι ὕπνῳ). However, since they had stayed fully awake (διαγρηγορήσαντες), they saw the glory of Jesus (δὲ εἶδαν τὴν δόξαν αὐτοῦ) and the 2 men (καὶ τοὺς δύο ἄνδρας) who stood with him (τοὺς συνεστῶτας αὐτῷ). This is another unique statement by Luke about Peter during the transfiguration, since the other synoptics did not mention this. Peter with his 2 companions were almost asleep, like they did later in the Garden of Gethsemane. However, Peter, James, and John stayed awake long enough to catch a glimpse of the glory of the transfigured Jesus, Moses, and Elijah, as they watched them talk together. There was no indication what language was being spoken, but the assumption might be that it was Hebrew or Aramaic. Have you ever fallen asleep so that you missed an important event?