Cardio in the morning on an empty stomach

Because body fat is left unprotected at this time. It's weak, because insulin levels are at rock bottom in the morning. This is the ideal time to maximally oxidize fat with a cardio- based type of activity, tripling fat mobilization over already high resting values. Examples of this are: Slow sustained walking on a treadmill on an incline, slow hikes that last for an hour. People tend to do too much high intensity cardio ,which eats up or catabolizes muscle tissue, this is because glycogen, which is stored in the liver and muscle tissue, is the easiest and fastest form of energy for your body to derive and burn. Ideally your goal is to preferentially burn fat ,not muscle.