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 what I call 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.
Burning more fat isn't the only reason you should do your cardio early. The third benefit of morning workouts is the "rush" and feeling of accomplishment that stays with you all day long after an invigorating workout. Exercise can become a pleasant and enjoyable experience, but the more difficult or challenging it is for you, the more important it is to get it out of the way early. When you put off any task you consider unpleasant, it hangs over you all day long, leaving you with a feeling of guilt, stress and incompleteness (not to mention that you are more likely to "blow off" an evening workout if you are tired from a long day at work or if your pals try to persuade you to join them at the pub for happy hour.)
You might find it hard to wake up early in the morning and get motivated to workout. But think back for a moment to a time in your life when you tackled a difficult task and you finished it. Didn't you feel great afterwards? Completing any task, especially a physically challenging one, gives you a "buzz." When the task is exercise, the buzz is physiological and psychological. Physiologically, exercise releases endorphins in your body.
Endorphins are opiate-like hormones hundreds of times more powerful than the strongest morphine. Endorphins create a natural "high" that makes you feel positively euphoric! Endorphins reduce stress, improve your mood, increase circulation and relieve pain. The "high" is partly psychological too. Getting up early and successfully achieving a small goal kick starts your day and gives you feelings of completion, satisfaction and accomplishment. For the rest of the day you feel happy and you feel less stress knowing that the most difficult part of the day is behind you.
So, you say you're not a morning person? Take heart; neither am I. I can sleep in like you wouldn't believe! But I get up anyway because I know the effort is worth the results. When I have a bodybuilding goal that I am clearly focused on, such as reaching 4% or 5% body fat for a competition, I'm on my Stairmaster for 45 minutes every morning at the crack of dawn without fail. Sure it's a challenge at first, but you know what? After a few short weeks, It's no longer a chore and I'm "in the groove" - and you will be too. Just try it.
Make a commitment to yourself to do it for just 21 days. Once those 21 days have gone by, you'll already be leaner and you'll be on your way to making morning workouts a habit that's as natural as brushing your teeth or taking a shower. Once you start getting used to feeling that buzz, you'll become "positively addicted" to it. The more you do it, the more you'll want to do it. Before you know it, early morning cardio will your new habit; you'll be leaner, your metabolism will be faster and you'll feel fantastic all day long!