Happy, when you workout will depend on your goals and the type of working out you're doing. Fat loss responds best to AM Cardio on an Empty Stomach. Strength training with a goal of muscle growth is best performed in the afternoons when you are at your strongest - it can however, also be done in the morning on an empty stomach. Many find this too difficult though.

You should also keep in mind that when you do your workouts are not as important as finding a time that works best for you - it is more important that you do the workouts. Morning workouts are recommended because statistically people who get up early to fit in their exercise tend to stick with their programs. Finding an exercise plan that works with your lifestyle should be one of your criteria.

HTH
Nat