No conclusive research exists on whether it’s better to exercise at the beginning or the end of the day. What IS proven is that working out on a consistent basis offers tremendous health benefits (lower rates of heart-disease, hypertension, stress, depression, etc.) not to mention looking and feeling great.
Many people work out in the morning because they feel that they are more committed at that time — they find too many excuses not to do it at the end of the day.
On the other hand, I teach exercise classes in the evenings at 6 p.m. or 6:30 p.m. because I found that they are more popular than the morning times. People who work often prefer to exercise on their way home from the office because they can build it into their schedule and it helps them unwind.
It depends more on your lifestyle than the time of day. Remember, though, that doing aerobic activity late in the evenings may interfere with your sleep, and if you’re too tired the next day, you probably won’t work out at all.