Adam Clarke on Psalms 104:20

Thou makest darkness - It is not the design of God that there should be either constant darkness or constant light. That man may labor, he gives him, by means of the sun, the light of the day; and that he may rest from his labor, and get his strength recruited, he gives him night, and comparative darkness. And as it would not be convenient for man and the wild beasts of the forest to collect their food at the same time, he has given the night to them as the proper time to procure their prey, and the day to rest in. When Man labors, They rest; when Man rests, They labor.