According to Strong's Concordance, agrupneo, the Greek word translated as watch in verse 36 means "to be sleepless, i.e. keep awake." Frequently, when the Bible mentions being asleep or tells us to wake up, it refers to our spiritual state (Matthew 25:5; Romans 13:11; I Thessalonians 5:6-8). Instead of "watch," some Bible versions use words such as, "don't go to sleep at the switch" (The Message), "be always on the watch" (NIV), "be ready all the time" (New Century Version), "keep awake" (Amplified Bible), "keep on the alert" (NASB), "stay awake" (ESV), "keep a constant watch" (Living Bible), and "beware of slumbering" (The New Testament in Modern Speech). This is a call to the spiritual, not the physical.
Just over two decades ago, an elderly man named Herbert Armstrong cried out, "Wake up!" and he was not talking about any other waking up than a spiritual one. Because we did not heed his warning then, the church has experienced twenty years of apostasy and scattering. If we do not wake up eventually, God has a three-and-a-half-year plan guaranteed to get our attention.
In our former association, we obeyed the instructions in Luke 21:7-33 to watch world events, but we did not closely follow Christ's commands in Luke 21:34-36 to guard our spiritual condition, hence the scattering. Interestingly, the condition of the church at that time mirrors how Luke 21:36 was generally applied—physical rather than spiritual.
It is always a good practice to allow the Bible to interpret itself rather than adding extra-biblical interpretations (II Peter 1:20). Because the Bible uses sleep and waking from sleep as spiritual metaphors, why would we want to add another meaning to the "watch" of Luke 21:36? That would be walking on shaky ground (Deuteronomy 4:2; 12:32), and we want to avoid repeating past error.
To emphasize that "watch" in Luke 21:36 is all about the spiritual and not about the physical, notice how agrupneo is used in its only other appearances in the New Testament:
Ephesians 6:18: ". . . praying always with all prayer and supplication in the Spirit, being watchful to this end with all perseverance and supplication for all the saints." (The context of this verse is putting on the whole armor of God—definitely a spiritual exercise.)
Hebrews 13:17: Obey those who rule over you, and be submissive, for they watch out for your souls, as those who must give account. Let them do so with joy and not with grief, for that would be unprofitable for you. (The ministry's first priority is the spiritual health of called-out Christians.)
These facts lead to the conclusion that "watch" in Luke 21:36 has little, or perhaps even nothing, to do with watching world events. A careful reading shows that the "watch" of Luke 21:36 is only minimally directing us to watch world events. Overemphasizing that meaning of this verse has overshadowed its real message, perhaps the most important survival instructions Jesus gives to Christians living at the end.