From The Gospel of Mark, by Mary Healy, commenting on Mark 13:33-37
Be watchful! is the refrain that has been repeated throughout the discourse (vv. 5, 9, 23). This time Jesus adds, Be alert! (or “stay awake!”), as he will again urge the disciples during his agony in the garden (14:34). The fact that the disciples do not know when the time will come means that they are to live in a state of constant watchfulness….
Be on the watch (grēgoreō) is another verb that means to stay awake and be on the lookout, part of the duty of prophets (Lam 2:19; Ezek 3:17; Hab 2:1). Again Jesus emphasizes that they do not know when he is coming. Lord (Greek kyrios) refers to Jesus in his lordship over the house of God—both the temple of the old covenant and the Church of the new covenant.
He may appear at any of the four divisions of the night, in Roman reckoning. Jesus is speaking of his sudden and unexpected coming at the end of time, when he will judge his disciples for how they have exercised their authority in the Church. But Mark also links this warning to Jesus’ passion by structuring the passion narrative precisely in terms of these four night watches: evening (Mark 14:17), midnight (implied in 14:32–65), cockcrow (14:72), and morning (15:1). Jesus warns that he may come suddenly and find them sleeping—which is just what will happen during his agony in Gethsemane (14:37–41).
To be asleep signifies spiritual torpor and self-indulgence (Rom 11:8; 1 Thess 5:6–8); to be awake is to be alive in faith (Rom 13:11; Eph 5:14). The trial in Gethsemane is the beginning of the trial that will last throughout the whole age of the Church, in which Jesus’ followers are called to be constantly alert and attentive to the presence of their Lord.
The final verse affirms that his warning is directed not only at the four who are privy to this discourse (13:3), but to all his disciples for all time: Watch! There is no room for complacency in the Christian life.