Normal human intake of water is excreted through urine, evaporation of water through the skin, and also through the breathing of water from the lungs. When a patient wakes up in the middle of the night, because the patient often drinks less water before going to bed, there will be a decrease in circulating water, and at this time, there will be less humidity in the gas exhaled from the lungs, and the performance of dry mouth will appear. If the patient’s urine volume is normal, the number of nighttime urination is normal, and the water in the body is reduced, the water discharged through the lungs is reduced, and the exhaled gas also takes away part of the water in the oral pharynx and is discharged from the body. If the patient breathes excessively, carbon dioxide will be expelled from the mouth and combined with water to form carbonic acid, which will cause dry mouth and even a feeling of bad taste in the mouth.