35 But for us, beloved brethren, besides the hours of prayer observed of old, both the times and the sacraments have now increased in number. For we must also pray in the morning, that the Lord's resurrection may be celebrated by morning prayer. And this formerly the Holy Spirit pointed out in the Psalms, saying, “My King, and my God, because unto You will I cry; O Lord, in the morning shall You hear my voice; in the morning will I stand before You, and will look up to You.” And again, the Lord speaks by the mouth of the prophet: “Early in the morning shall they watch for me, saying, Let us go, and return unto the Lord our God.” Also at the sunsetting and at the decline of day, of necessity we must pray again.
For since Christ is the true sun and the true day, as the worldly sun and worldly day depart, when we pray and ask that light may return to us again, we pray for the advent of Christ, which shall give us the grace of everlasting light. Moreover, the Holy Spirit in the Psalms manifests that Christ is called the day. “The stone,” says He, “which the builders rejected, has become the head of the corner. This is the Lord's doing; and it is marvellous in our eyes. This is the day which the Lord has made; let us walk and rejoice in it.” Also the prophet Malachi testifies that He is called the Sun, when he says, “But to you that fear the name of the Lord shall the Sun of righteousness arise, and there is healing in His wings.” But if in the Holy Scriptures the true sun and the true day is Christ, there is no hour excepted for Christians wherein God ought not frequently and always to be worshipped; so that we who are in Christ— that is, in the true Sun and the true Day— should be instant throughout the entire day in petitions, and should pray; and when, by the law of the world, the revolving night, recurring in its alternate changes, succeeds, there can be no harm arising from the darkness of night to those who pray, because the children of light have the day even in the night. For when is he without light who has light in his heart? Or when has not he the sun and the day, whose Sun and Day is Christ?
Source: The Treatises of Cyprian (New Advent)