every night when i come home from work i'm so exauhsted but i just sit at a desk why is this?

Answer 1

because you are not moving around. you need to get up from time to time and walk around and get some air. Can you walk on your lunch? just move more.

Answer 2

It's idle work. You're sitting there and not exercising your muscles. I work at a desk job myself and the days seem longer and more tiring than when I did some physical labor in my youth.