1 This psalm has the title, “For David himself, when he was in the desert of Idumæa.” By the name of Idumæa is understood this world. For Idumæa was a certain nation of men going astray, where idols were worshipped. In no good sense is put this Idumæa. If not in a good sense it is put, it must be understood that this life, wherein we suffer so great toils, and wherein to so great necessities we are made subject, by the name of Idumæa is signified. Even here is a desert where there is much thirst, and you are to hear the voice of One now thirsting in the desert. But if we acknowledge ourselves as thirsting, we shall acknowledge ourselves as drinking also. For he that thirsts in this world, in the world to come shall be satisfied, according to the Lord's saying, “Blessed are they that hunger and thirst after righteousness, for the same shall be satisfied.” Therefore in this world we ought not to love fullness. Here we must thirst, in another place we shall be filled. But now in order that we may not faint in this desert, He sprinkles upon us the dew of His word, and leaves us not utterly to dry up, so that there should not be in our case any seeking of us again, but that we may so thirst as that we may drink. But in order that we may drink, with somewhat of His Grace we are sprinkled: nevertheless we thirst. And what says our soul to God?
2. “God, my God, unto You from the light I watch”. What is to watch? It is, not to sleep. What is to sleep? There is a sleep of the soul; there is a sleep of the body. Sleep of body we all ought to have: because if sleep of body is not taken, a man faints, the body itself faints. For our frail body cannot long sustain a soul watching and on the stretch on active works; if for a long time the soul shall have been intent on active pursuits, the body being frail and earthly holds her not, sustains her not for ever in activity, and faints and falls. Therefore God has granted sleep to the body, whereby are recruited the members of the body, in order that they may be able to sustain the soul watching. But of this let us take heed, namely, that our soul herself sleep not: for evil is the sleep of the soul. Good is the sleep of the body, whereby is recruited the health of the body. But the sleep of the soul is to forget her God. Whatsoever soul shall have forgotten her God, sleeps. Therefore the Apostle says to certain persons that forgot their God, and being as it were in sleep, did act the follies of the worship of idols— the Apostle, I say, says to certain persons, “Rise, you that sleepest, and rise up from the dead, and Christ shall enlighten you.” Was the Apostle waking up one sleeping in body? Nay, but he was waking a soul sleeping, inasmuch as he was waking her, in order that she might be lightened by Christ. Therefore as to these same watchings says this man, “God, my God, unto You from the light I watch.” For you would not watch of yourself, unless there should arise your Light, to wake you from sleep. For Christ lightens souls, and makes them to watch: but if His light He takes away, they slumber. For for this cause to Him there is said in another psalm, “Lighten my eyes, that I may never slumber in death.”...
3. “My soul has thirsted for You”. Behold that desert of Idumæa. See how here he thirsts: but see what good thing is here, “Hath thirsted for You.” For there are they that thirst, but not for God. For every one that wills anything to be granted to him, is in the heat of longing; the longing itself is the thirst of the soul. And see ye what longings there are in the hearts of men: one longs for gold, another longs for silver, another longs for possessions, another inheritance, another abundance of money, another many herds, another a wife, another honours, another sons. You see those longings, how they are in the hearts of men. All men are inflamed with longing, and scarce is found one to say, “My soul has thirsted for You.” For men thirst for the world: and perceive not themselves to be in the desert of Idumæa, where their souls ought to thirst for God....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)