The majority of people who exercise consistently do so early in the day. It is easier to form the exercise habit through morning exercise.
• Fewer distractions and schedule interruptions.
• Can make time for exercise by getting up a bit earlier.
• Raises your heart rate and metabolism to burn more calories earlier in the day.
• Gives a feeling of physical energy for hours.
• Improves your mental acuity for hours.
• Cooler temperatures in summer.
• Air pollution is lowest in the morning.
• The body adjusts to your exercise time, so if you are training for a morning sports event, train in the morning.
This was taken from an About.com article, I hope it helps!