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.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)