Compline (completorium) is so-called because at that point our daily consumption of food and drink, which are necessarily consumed for the sustenance of the body, together with ordinary speech, is complete (completur). And thus monks, in accordance with Saint Benedict's rule, see to the closing of their mouths and keep them distant from ordinary conversation from the time of this office until they return to work again. And it is not inappropriate that this time of night, in which all things are silent, can be called quietude. Among those who divide the night into seven parts, the first part is called twilight, the second vespers, and the third quietude, which we are now discussing. And I think that it is so-called because, at this moment, all things are quiet.
In some sense this office corresponds to that commendation through which man commends himself to God when he leaves this world. Sleep is the image of death. And just as the mind of the dead man is withdrawn from mortal things and given over to forgetfulness of the suffering of this world, so in some sense is the spirit of the sleeping man withdrawn from its usual thoughts and from every temporal occupation.
In the four Psalms we commend the four elements of our body to the Lord, as the vesicle makes clear, saying: "Keep me, Lord, the apple of your eye. Protect me under the shadow of your wings." Everyone—even those with tenuous understanding—recognizes how many more external dangers can befall a man when he is asleep than when he is awake
Augustine, writing on a verse from the first Psalm that is sung during Compline, explains this: "'In peace in the selfsame I will sleep and I will take rest.' For it is right that such people hope for all manner of mental withdrawal from mortal affairs, and for forgetfulness of the miseries of this world.
The second Psalm continues up to the verse in which Christ laid down his spirit on the cross. In that verse, we desire that we be brought into conformity with his sleep — that is, that our members rest while our heart is vigilant.
The third Psalm is filled with words of prayers that request the Lord's protection. In Roman usage it is also sung on Good Friday after the reading, where (according to Hosea) our mortification after the example of Christ is revealed, together with our resurrection after two days. Through this Psalm, the author of the Office advises us, in accordance with its words, that our mind should be intent on beseeching God amid all our dangers and difficulties; and thus, because our sleep has some likeness to the sleep of those who have left this world under the Lord's protection, the same Psalm is recited at Compline.
In some sense the fourth Psalm also recalls the intention of those who are in difficulty and nevertheless pass from this difficulty to peace, saying: "In the nights lift up your hands to the holy places." Thus Augustine in his treatise on the same Psalm: "For the night is a sorrowful thing, and the day is a joyful thing." And after a little: "Therefore, 'Bless the Lord.' When? At night. When Job blessed him, what a sorrowful night it was! All his possessions were taken away; the sons who had kept them were taken away. What a sorrowful night! But let us see if he does not give his blessing during the night: 'The Lord gave, and the Lord has taken away. As it has pleased the Lord, so was it done. Blessed be the name of the Lord. '"+
The vesicle that follows, as we said above, clarifies the full task of the office—namely, a request for the Lord's protection amid the dangers of the night. And the subsequent hymn of Saint Simeon reveals the peace that the soul of the one praying desires—namely, that it may rest without any disturbance from this world. This is what Simeon prayed for when he wished to pass from this life to the other life, and said: "Now dismiss your servant in peace, Lord, according to your word."