Glycogen, otherwise known as sugar, within the muscles and liver decrease in the night as you sleep. Conventional wisdom suggests that exercising on an empty stomach would promote fat burning and weight loss.
A study from the University of Padua in Italy found the opposite to be true: physical activity after a light meal WORKED BEST best for losing body fat. Researchers studied fat metabolism in 8 males who ran on a treadmill at 65% of their maximum heart rate for 36 minutes on two different occasions - either without eating and after a light breakfast. Metabolism was higher when the subjects ate during and 12 hours AFTER exercise. The amount of fat burned was also higher after exercise and in the fed condition.
Consume a light meal about 30 to 60 minutes before exercise, if your goal is to lose weight. (International Journal Sports Nutrition Exercise Metabolism, 21: 48-54, 2011)