Fatigue is a major problem.

Most people use caffeinated energy drinks to get more energy. But there's a cost. First you're jacked up. Then you crash. When you're exhausted, your brain, muscles, and cells cry out for more oxygen. As blood travels through the lungs, your red blood cells act as magnets for oxygen. This oxygen-rich blood releases oxygen wherever it is needed. The more red blood cells you have in your body, the greater your oxygen-carrying capacity.