for maximum fat loss the time of day means everything !
some studies have suggested that up to 300% more fat is burned when cardio is done in a fasted, glycogen-depleted state.
If you do cardio immediately after eating a meal, you'll still burn fat, but you'll burn less of it because you'll be burning off the carbohydrates you ate first. You always burn a combination of fat and carbohydrate for fuel, but depending on when you exercise, you can burn a greater proportion of fat relative to carbohydrate. If doing cardio first thing in the morning is not an option for you, then the second best time to do it would be immediately after weight training. Lifting weights is anaerobic (carbohydrate-burning) by nature, and therefore depletes muscle glycogen. That's why a post lifting cardio session has a similar effect as morning cardio on an empty stomach.
The second benefit you'll get from early morning cardio sessions is called the "afterburn" effect. When you do a cardio session in the morning, you not only burn fat during the session, but you also continue to burn fat at an accelerated rate after the workout. Why? Because an intense session of cardiovascular exercise can keep your metabolism elevated for hours after the session is over. If you do cardio at night, you will still burn fat during the session, so you definitely benefit from it. However, nighttime cardio fails to take advantage of the "afterburn" effect because your metabolism drops like a ton of bricks as soon as you go to sleep. While you sleep, your metabolic rate is slower than any other time of the day.