Whether it's early in the morning or late at night, countless people all over the world depend on caffeine for a boost of energy. That includes athletes, and many reach for a caffeinated energy drink. Researchers know that modest use of caffeine does have value for some exercise, but any more than the level found in a few cups of coffee or multiple energy drinks can exceed what’s valuable for exercise. Physician John MacKnight explains how it can be easy – and dangerous – for athletes to take in too much caffeine.