However, the "timing is everything" rule also applies wh Absolute Keto n using exercise to lose body fat. Exercising at the right time is just as important as doing the right exercises. When is the best time to do fat-burning exercises? In particular, the ideal time to do your aerobic exercise is in the morning. The reason is that your body's level of glycogen (stored carbohydrates) is extremely low then. That's because you've likely been fasting for about 12 hours since dinnertime.