John Calvin Reformed @genevareformer

Here Daniel relates the tardy repentance of the king, because although he was in the greatest grief, yet he did not correct his fault. And this occurs to many who are not hardened by contempt of God and their own depravity; they are drawn aside by others, and are dissatisfied with their own vices, while they still indulge in them.

Matthew Henry Presbyterian @wholebiblehenry

Here is, I. The melancholy night which the king had, upon Daniel's account, Dan 6:18. He had said, indeed, that God would deliver him out of the danger, but at the same time he could not forgive himself for throwing him into the danger; and justly might God deprive him of a friend whom he had himself used so barbarously.

Commenting on Daniel 6:18-24

John Gill Reformed Baptist @doctorgill

Then the king arose very early in the morning,.... Or, "in the morning with light" (i) as soon as ever light appeared, or the day broke: the word for morning is doubled, and one of the letters in it is larger than usual; and all which denote not only his very great earliness in rising, but his earnestness and solicitude for Daniel, to know whether...