Why is there mental fatigue? Can a soul be empty? ...

And why not? If there is no prayer, it will be empty and tired. Holy Fathers proceed as follows. The man is tired, there is no strength to pray, he says to himself: “But maybe your tiredness is from the demons,” he gets up and prays. And a person has strength. So the Lord arranged. In order for the soul not to be empty and to have strength, you must accustom yourself to the Jesus prayer - "Lord, Jesus Christ, Son of God, have mercy on me, a sinner "

Archimandrite Ambrose (Yurasov)