12 “If I have remembered You upon my bed, in the dawnings I did meditate on you: because You have become my helper”. His “bed” he calls his rest. When any one is at rest, let him be mindful of God; when any one is at rest, let him not by rest be dissolved, and forget God: if mindful he is of God when he is at rest, in his actions on God he does meditate. For the dawn he has called actions, because every man at dawn begins to do something. What therefore has he said?
If therefore I was not mindful on my bed, in the dawn also I did not meditate on You. Can he that thinks not of God when he is at leisure, in his actions think of God? But he that is mindful of Him when he is at rest, on the Same does meditate when he is doing, lest in action he should come short. Therefore he has added what? “Because You have become my helper.” For unless God aid our good works, they cannot be accomplished by us. And worthy things we ought to work: that is, as though in the light, since by Christ showing the way we work.
Whosoever works evil things, in the night he works, not in the dawn; according to the Apostle, saying, “They that are drunken, in the night are drunken; and they that sleep, in the night do sleep; let us that are of the day, be sober.” He exhorts us that after the day we should walk honestly: “As in the day, honestly let us walk.” And again, “You,” he says, “are sons of light, and sons of day; we are not of night nor of darkness.” Who are sons of night, and sons of darkness?
They that work all evil things. To such a degree they are sons of night, that they fear lest the things which they work should be seen....No one therefore in the dawn works, except him that in Christ works. But he that while at leisure is mindful of Christ, on the Same does meditate in all his actions, and He is a helper to him in a good work, lest through his weakness he fail. “And in the covering of Your wings I will exult.” I am cheerful in good works, because over me is the covering of Your wings.
If you protect me not, forasmuch as I am a chicken, the kite will seize me. For our Lord Himself says in a certain place to that Jerusalem, a certain city, where He was crucified: “Jerusalem,” He says, “Jerusalem, how often I have willed to gather your sons, as though a hen her chickens, and you would not.” Little ones we are: therefore may God protect us under the shadow of His wings. What when we shall have grown greater? A good thing it is for us that even then He should protect us, so that under Him the greater, always we be chickens.
For always He is greater, however much we may have grown. Let no one say, let Him protect me while I am a little one: as if sometime he would attain to such magnitude, as should be self-sufficient. Without the protection of God, nought you are. Alway by Him let us desire to be protected: then always in Him we shall have power to be great, if always under Him little we be. “And in the covering of Your wings I will exult.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)