The human body is designed to move and expend energy in order to survive. After a long hike, the body has expended a lot of energy and needs time to rest and recover before it can do it again. The muscles have been working hard for hours, the heart has been pumping blood at an increased rate, and the brain is processing all of this information. In addition, when you are hiking uphill your muscles work harder than when you are hiking downhill because gravity is pulling on them more forcefully. All of these factors combined make hiking tired!