I've read that it technically makes no difference what time you do your cardio at. Some people believe that doing cardio in the morning before breakfast is more beneficial than doing it later in the day. It might be that this works, to a certain extent. If you do it on an empty stomach in the morning, then what you'll be burning is most likely glycogen (stored glucose). The typical adult has anywhere from 700-1500 calories stored in glycogen. If you burn 80 calories an hour sleeping, then you should probably have no problem burning up the rest of it with a good workout. But I wonder what the point of this is? Why burn up your stored glucose? I say leave it alone. It's not like you're burning fat when you do it. Do your cardio whenever you want, but just do it. Calories in = calories available. Burning more calories will lead to fat loss, more or less. Exercising in the morning will increase your metabolism for the day, though, but I'm not sure by how much. I say do it whenever it's convenient. And I'm a fan of doing it after you've had some food, so you don't bonk. I think it's pointless to do it on an empty stomach.