4 And as if you should say, When shall we rise? We are ordered now to sit: when will be our rising? When the Lord's was. Look unto Him, who went before you: for if you heed not Him, “it is lost labour for you to rise before dawn.” When was He raised? When He had died. Hope therefore for your uplifting after your death: have hope in the resurrection of the dead, because He rose again and ascended. But where did He sleep? On the Cross. When He slept on the Cross, He bore a sign, yea, He fulfilled what had been signified in Adam: for when Adam was asleep, a rib was drawn from him and Eve was created; so also while the Lord slept on the Cross, His side was transfixed with a spear, and the Sacraments flowed forth, whence the Church was born.
For the Church the Lord's Bride was created from His side, as Eve was created from the side of Adam. But as she was made from his side no otherwise than while sleeping, so the Church was created from His side no otherwise than while dying. If therefore He rose not from the dead save when He had died, do you hope for exaltation save after this life? But that this Psalm might teach you, in case you should ask, When shall I rise? Perhaps before I have sat down? He adds, “When He has given His beloved sleep”.
God gives this when His beloved have fallen asleep; then His beloved, that is, Christ's, shall rise. For all indeed shall rise, but not as His beloved. There is a resurrection of all the dead; but what says the Apostle? “We shall all rise, but we shall not all be changed.” They rise unto punishment: we rise as our Lord rose, that we may follow our Head, if we are members of Him....Hope for such a resurrection; and for the sake of this be a Christian, not for the sake of this world's happiness.
For if you wish to be a Christian for the sake of this world's happiness, since He your Light sought not worldly happiness; you are wishing to rise before the light; you must needs continue in darkness. Be changed, follow your Light; rise where He rose again: first sit down, and thus rise, “when He gives His beloved sleep.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)