Though your metabolism IS constantly at work, it does slow down later in the day, especially if you are just dieting and not exercising. When you exercise your heart rate and metabolism both increase. In addition it is better to eat more meals and take in the same amount of calories because in doing so you keep your metabolism working. On the other hand if you eat less or worse starve yourself for several hours your metabolism slows down and potentially puts your body into a "starvation mode" where more insulin is released causing the body to store more fat. This is the most simple answer to this question.