As well as you can, eat well and get some exercise! It could save your life or the lives of others around you. Mid morning or afternoon fatigue is caused by insulin spikes through the day. Small portions, more often of protein and fresh greens. Cans of tuna, some nuts and dairy. Grab a green salad when you can or pack some preportioned if possible. Walk a good pace around your vehicle if nothing else or carry a jump rope with you for breaks. Drink your water and rest well when you can!
These tips also work well for those who are "stuck" at a desk all day or any other sedentary lifestyle.