Do you not know? 
   Have you not heard? 
The LORD is the everlasting God, 
   the Creator of the ends of the earth. 
He will not grow tired or weary, 
   and his understanding no one can fathom. 
He gives strength to the weary 
   and increases the power of the weak. 
Even youths grow tired and weary, 
   and young men stumble and fall; 
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 40:28-31 (NIV)