40 Although by His words, Your will be done, He surrendered the Apostles to the decision of His Father's will, in regard to the offense of the cup, that is, of His Passion, still He repeated His prayer a second and a third time. After that He said, Sleep on now, and take your rest. It is not without the consciousness of some secret reason that He Who had reproached them for their sleep, now bade them sleep on, and take their rest. Luke is thought to have given us the meaning of this command.
After He had told us how Satan had sought to sift the Apostles as it were wheat, and how the Lord had been entreated that the faith of Peter might not fail, he adds that the Lord prayed earnestly, and then that an angel stood by Him comforting Him, and as the angel stood by Him, He prayed the more earnestly, so that the sweat poured from His body in drops of blood. The Angel was sent, then, to watch over the Apostles, and when the Lord was comforted by him, so that He no longer sorrowed for them, He said, without fear of sadness, Sleep on now, and take your rest.
Matthew and Mark are silent about the angel, and the request of the devil: but after the sorrowfulness of His soul, the reproach of the sleepers, and the prayer that the cup may be taken away, there must be some good reason for the command to the sleepers which follows; unless we assume that He Who was about to leave them, and Himself had received comfort from the Angel sent to Him, meant to abandon them to their sleep, soon to be arrested and kept in durance.
Source: On the Trinity (New Advent)