Morning and evening are good times to exercise. Working with the biological clock increases the benefits of exercise. Exercising at night or in the afternoon is not advisable. After sundown, body's functions slow down and energy is spent in other activites like resting and repair. Exercising then will strain the body and will reduce the benefits of exercise. It will also harm the body as energy will now be required to cope with the increased activity. In the noon, energy is required to digest food. Exercise after 3-4 hours of a meal. Again, choose sustainable exercises and not a crash regimen at the gym.