When you run in the a.m. without eating breakfast, your pace will feel harder than it is, you won't be able to reach higher intensities, and you'll burn fewer calories as your body goes into conservation mode from the lack of fuel in your system. This all happens because, unless you're a midnight snacker, you've been fasting for nine to 12 hours between dinner and sunup. During this time, your muscle and liver glycogen and blood-glucose levels drop--all crucial energy sources that fuel your running. By eating shortly after you awake, you break the fast and begin to top off your energy stores. With a full tank, your body is then primed for exercise.