[Joemoplata]

When you exercise your metabolism increases during the length of exercise. The "harder" the exercise (this is relative to the person doing the exercise) the more the increase. The more the increase, the longer lasting it's affect on you since an increase in metabolism is a slowly decaying chemical reaction. Overdoing it can backfire, however, and cause a "crash". Same thing happens when you eat.

This is why if you want to feel like you have more energy throughout the day and promote a steady state of metabolic rate (this means you are using your calories efficiently) you should eat small meals every 3 to 4 hours or so and exercise when you can (the earlier in the day the better). This promotes what is known as a good "Circadian Rhythm".