Workaday world

I've worked manual jobs in my life. My first job was picking up trash near and at the paper mill in my hometown. I've made heating and air conditioning ducts in a factory. I've worked for a certain fast-food company, from slinging sliders to shift manager. I've had reason to be tired in those jobs, having worked and moved and been on my feet all day. The gig I have now leaves me sitting at a desk, in front of a PC all day. So why am I as tired at the end of the day now as I was standing over a hot grill? Is it the mental aspects of my job wear on me? How can I recover well for the next test?