For maximum weight loss you should exercise in the morning to burn the most amount of fat and here's why...
During the day your body's main source of energy is the carbohydrates that you get from eating your meals. As you sleep at night for 6+ hours your body uses up all those carbohydrates as energy for various bodily functions that go on while you sleep. When you wake up in the morning your body doesn't have any carbohydrates as energy to use and it will look to burn body fat instead for energy. For you to take advantage of this morning fat burning opportunity you have to exercise first thing in the morning. Don't eat breakfast because if you do... You'll just give your body some carbohydrates as a source of energy instead of the body fat that you want to burn for energy. Another great thing about working out first thing in the morning is that your metabolism gets revved up after your morning workout. Morning workouts keep your metabolism elevated throughout the day. An elevated metabolism throughout the day only means that you'll burn more calories and lose more weight. If you workout at night you may still burn fat while you workout but as soon as you go to sleep your metabolism will slow down and you'll miss out on all the extra fat that you can burn during the day if you had exercised in the morning. When you sleep your metabolic rate is always at its slowest.