The main reason is that the venous circulation of human lower limbs is affected to a certain extent, such as patients suffering from deep vein thrombosis of lower limbs or superficial varicose veins of lower limbs, the human body is more in an upright state during the day, and the blood flow back to the human body will be affected under the effect of gravity, so a large amount of blood will accumulate in the human lower limbs, resulting in swelling of the lower limbs, including the feet, during the day. During nighttime sleep, as a result of lying down, a large amount of blood will flow back to the human heart, which means that there is no gravity, and the blood pressure in the human lower limbs will be relatively reduced at this time, so the patient will experience a gradual disappearance of swelling at night. Most of the time, this is due to the presence of deep vein thrombosis or superficial varicose veins in the lower extremities.