11 Mark also records these passages, introducing them quite in the same method and succession. Some of the sentences, however, are given with greater brevity by him, and others are somewhat more fully explained. These sayings of our Lord, indeed, may seem in one portion to stand in some manner of contradiction to each other as they are presented in Matthew's version. I refer to the fact that [it is stated there that] He came to His disciples after His third prayer, and said to them, “Sleep on now, and take your rest: behold, the hour is at hand, and the Son of man shall be betrayed into the hands of sinners.
Rise, let us be going: behold, he is at hand that shall betray me.” For what are we to make of the direction thus given above, “Sleep on now, and take your rest,” when there is immediately subjoined this other declaration, “Behold, the hour is at hand,” and thereafter also the instruction, “Arise, let us be going”? Those readers who perceive something like a contradiction here, seek to pronounce these words, “Sleep on now, and take your rest,” in a way betokening that they were spoken in reproach, and not in permission.
And this is an expedient which might quite fairly be adopted were there any necessity for it. Mark, however, has reproduced these sayings in a manner which implies that after He had expressed himself in the terms, “Sleep on now, and take your rest,” He added the words, “It is enough,” and then appended to these the further statement, “The hour has come; behold, the Son of man shall be betrayed.” Hence we may conclude that the case really stood thus: namely, that after addressing these words to them, “Sleep on now, and take your rest,” the Lord was silent for a space, so that what He had thus given them permission to do might be [seen to be] really acted upon; and that thereafter He made the other declaration, “Behold the hour has come.”
Thus it is that in Mark's Gospel we find those words [regarding the sleeping] followed immediately by the phrase, “It is enough;” that is to say, “the rest which you have had is enough now.” But as no distinct notice is introduced of this silence on the Lord's part which intervened then, the passage comes to be understood in a forced manner, and it is supposed that a peculiar pronunciation must be given to these words.
Source: The Harmony of the Gospels (New Advent)