Why Do I Get Sleepy While Exercising?

Because all of your blood is going to your muscles instead of your head:)

While exercising we use lot of energy and that energy is obtained from oxidative phosphorylation. Oxidative phosphorylation requires oxygen for ATP / energy production. As we exercise we require more oxygen. If we can't get more and sufficient oxygen then brain indirectly give massage to take rest to fulfill oxygen requirment. That is aquired by stimulating enzymatic activity in the mesencephalon. That is the reason why you get sleepy while exercising? To avoid it you do exercise in open space where you can get more oxygen.