I am tired today - my boy is waking in the night at the moment, which means I am too, and it's taking it's toll. It's true that if we don't rest we will be tired, sluggish, probably grumpy and have a lack of energy.
The same is true of our spiritual life. When we don't 'rest' in God the same occurs in our spirit. We need to wait on God so that we can be renewed in our spirit. Do you feel tired and heavy laden - rest in God and bring your burdens to him. Let him be your vitamin - he will pick you up!
Jesus said, "Come to me those of you that are tired and heavy-laden, and I will give you rest for your souls". Simply...come.
I know I need to do this now. Every blessing to you this day.