When you are tired, turn to God and say: "Master of the World! With my own strength I am too tired to serve you further; but if You want me to continue, You have to give me Your strength, as it says, 'Those who hope in the Lord will receive new strength' [Isaiah 40:31]). Blessed are You, who gives strength to the weary." Then, trusting in God's strength, continue Your Divine service. . . .
Or when you are tired you can say: "Master of the World! With my own strength I am too tired to continue. Let me then serve you in the way of a sacrifice."
We usually think that we are to do something when we seem to have adequate energy to do it; but recognizing that we do not have the energy, we can do the service anyway, acting from the special energy source of self-sacrifice. Going beyond yourself in this way is a true sign of devotion to God.
— Yitzhak Buxbaum in Jewish Spiritual Practices