when you do things that are strenuous to your body your digestive system actually slows down. More of your blood oxygen is used for musculoskeletal movement hence the slowing down of the metabolical processes of your digestive system. This leads to a decrease in appetite. This is what happens while your working. Also while you aren't working your body probably has burnt enough carbohydrates to start lipid metabolism which lasts longer than carbohydrate and protein metabolism so you don't feel as hungry. I work for a moving company during the summer and have the same thing often times I wouldn't eat lunch and rarely ate more than once a day.