God uses tired people who trust Him more than their own strength. - J.I. Packer

But those who hope in the Lord will renew their strength. They will soar on wings like eagles; they will run and not grow weary, they will walk and not be faint. - Isaiah 40:31

So we do not lose heart. Though our outer self is wasting away, our inner self is being renewed day by day. - 2 Corinthians 4:16

When you're weak, God is strong.