As if you needed another reason to hate the gym, it now turns out that exercise can exhaust not only your muscles, but also your eyes. Fear not, however, for coffee can perk them right up again

. During vigorous exercise, our muscles tire as they run out of fuel and build up waste products. Muscle performance can also be affected by a phenomenon called "central fatigue," in which an imbalance in the body's chemical messengers prevents the central nervous system from directing muscle movements effectively. It was not known, however, whether central fatigue might also affect motor systems not directly involved in the exercise itself—such as those that move the eyes. To find out, researchers gave 11 volunteers a carbohydrate solution either with a moderate dose of caffeine—which is known to stimulate the central nervous system—or as a placebo without, during 3 hours of cycling. After exercising, the scientists tested the cyclists with eye-tracking cameras to see how well their brains could still control their visual system. The team found that exercise reduced the speed of rapid eye movements by about 8%, preventing their ability to capture new visual information. The caffeine—the equivalent of two strong cups of coffee—was sufficient to counteract this effect, with some cyclists even displaying increased eye movement speeds, the team reports today in Scientific Reports. So it might be a good idea to get someone else to drive you home after that marathon.