As a dream when one awaketh - Their prosperity is like the visions of a dream; the reality is seen when one awakes. A man in a dream may imagine that he is a king; that he dwells in a palace; that he is surrounded by flatterers and courtiers; that he walks in pleasant groves, listens to the sounds of sweet music, sits down at a table loaded with the luxuries of all climes, and lies upon a bed of down. He may awake only to find that he is encompassed with poverty, or that he is on a bed of languishing, or that he is the miserable tenant of a hovel or a dungeon. The reality is when he awakes. So it is in regard to our present condition on earth. The reality is seen when the dream - the gorgeous dream - of life is over.

So, O Lord, when thou awakest - The Hebrew expression here - בעיר bā‛ı̂yr - occurs in more than fifty other places in the Scriptures, and is in all these places translated "in the city." This interpretation, however, would be quite unmeaning here, and the probability is that the expression is a form of the verb עור ‛ûr, "to awake, to arouse;" and the idea is not, as in our version, that of "God's" awaking as if he had been asleep, but it refers to the dreamer when he shall awake. It is, literally, in the awaking; that is, when the dream is over.

Thou shalt despise their image - The image that floated before their imaginations in the dream of life. Thou wilt pay no attention to it; there is no reality in it; it will at once vanish. In the future world, God will pay no regard to the dreams of human life, to the outward show, to the appearance; but the affairs of eternity will be regulated by what is real - by that which constitutes the character of the man. By that, and not by the vain dreams of the world, will the destiny of people be determined. We are to look at "that" in determining the question about the government of God, and not at what "appears" in the brief dream of life.

Psalm 73:20 Parallel Commentaries

Of a Low Estimation of Self in the Sight of God
I will speak unto my Lord who am but dust and ashes. If I count myself more, behold Thou standest against me, and my iniquities bear true testimony, and I cannot gainsay it. But if I abase myself, and bring myself to nought, and shrink from all self-esteem, and grind myself to dust, which I am, Thy grace will be favourable unto me, and Thy light will be near unto my heart; and all self-esteem, how little soever it be, shall be swallowed up in the depths of my nothingness, and shall perish for ever. …
Thomas A Kempis—Imitation of Christ

Covenanting Adapted to the Moral Constitution of Man.
The law of God originates in his nature, but the attributes of his creatures are due to his sovereignty. The former is, accordingly, to be viewed as necessarily obligatory on the moral subjects of his government, and the latter--which are all consistent with the holiness of the Divine nature, are to be considered as called into exercise according to his appointment. Hence, also, the law of God is independent of his creatures, though made known on their account; but the operation of their attributes …
John Cunningham—The Ordinance of Covenanting

Cæsarius of Arles.
He was born in the district of Chalons-sur-Saone, A. D. 470. He seems to have been early awakened, by a pious education, to vital Christianity. When he was between seven and eight years old, it would often happen that he would give a portion of his clothes to the poor whom he met, and would say, when he came home, that he had been, constrained to do so. When yet a youth, he entered the celebrated convent on the island of Lerins, (Lerina,) in Provence, from which a spirit of deep and practical piety …
Augustus Neander—Light in the Dark Places