First, it takes at least 2-3 hours for aspirin enter the intestinal tract in the fasting state. And it takes about 3 hours for the stomach to empty after a meal, so that there is no good point in time to meet this requirement either before breakfast, and between three meals, while bedtime is generally more than 3 hours from dinner, which can be considered fasting, and the gastric pH should be sufficient to ensure acidity. Second, most of the cardiovascular adverse events occur in the early morning and morning, and oral aspirin to the absorption into the blood takes more than 3-4 hours to play a role, breakfast dosing can not well cover the morning peak period, and bedtime dosing can just enter the peak of blood concentration in the early morning, so that aspirin more effective.