The exercise time should be from 3 pm to 9: 30 pm, because the air at this time is fresh, and after a long night, the air in the morning is turbid. If you must exercise in the morning, you must wait until the sun rises. Because flowers and trees emit carbon dioxide and absorb oxygen at night, they emit oxygen and absorb carbon dioxide through the photosynthesis of sunlight. Therefore, if you want to exercise in the morning, you should choose the most suitable after the sun rises.