Musar su Salmi 27:1

Another interpretation, also quoted there, is that when David composed Psalm 27,1: "The Lord is my light and my help; whom should I fear?" He meant that just as Torah is dear to G–d so are those that study Torah very dear to G–d. Anyone who is steeped in Torah study need not worry about all the destructive forces that abound on this earth. He is able to subjugate all these destructive forces and consign them to the Deep. When night falls the gates of another world are opened. Dogs and wild asses roam the streets and while their owners are asleep in their beds, the destructive forces are allowed full reign. During that time, the souls of the righteous ascend to Heaven to enjoy its atmosphere. When the North Wind becomes active at midnight, forces of sanctity become active in the world, as has been explained in several places. Hail to the man who arises from his sleep at that hour and busies himself with Torah study. Due to his Torah study he is able to expel all the forces of destruction that are around at that time. He subjugates the קליפה of the חמור and consigns it to a sphere where the pollutants prevail. Hence Issachar whose toil is with Torah, succeeds in banning the חומר, [play on words, “non is symbol of all that is physical. Ed.] The חמור, cause of the ascendancy of the destructive elements is consigned by Isaachar to the nether regions גרם, the reservoir of the pollutants.