10 Now, how can a man who cannot by his own strength and labour come to Jesus, at the same time avoid all sins? And avoid them perpetually, and claim for himself a name which belongs to the might of God? For if He and I are both without sin, what difference is there between me and God? One more proof only I will adduce, that I may not weary you and my hearers. Sleep was removed from the eyes of Ahasuerus, whom the Seventy call Artaxerxes, that he might turn over the memoirs of his faithful ministers and come upon Mordecai, by whose evidence he was delivered from a conspiracy; and that thus Esther might be more acceptable, and the whole people of the Jews escape imminent death.
There is no doubt that the mighty sovereign to whom belonged the whole East, from India to the North and to Ethiopia, after feasting sumptuously on delicacies gathered from every part of the world would have desired to sleep, and to take his rest, and to gratify his free choice of sleep, had not the Lord, the provider of all good things, hindered the course of nature, so that in defiance of nature the tyrant's cruelty might be overcome. If I were to attempt to produce all the instances in Holy Writ, I should be tedious.
All that the saints say is a prayer to God; their whole prayer and supplication a strong wrestling for the pity of God, so that we, who by our own strength and zeal cannot be saved, may be preserved by His mercy. But when we are concerned with grace and mercy, free will is in part void; in part, I say, for so much as this depends upon it, that we wish and desire, and give assent to the course we choose. But it depends on God whether we have the power in His strength and with His help to perform what we desire, and to bring to effect our toil and effort.
Source: Against the Pelagians (New Advent)