Yawning may be caused by lack of oxygen in the brain, lack of sleep, metabolic encephalopathy and other reasons. 1. Lack of oxygen in the brain: yawning may be caused by lack of oxygen in the brain tissue because brain cells are very sensitive to oxygen, and if the brain tissue is mildly deprived of oxygen, the symptom of yawning will occur. 2. Lack of sleep: If the recent work pressure is relatively large and the sleep is insufficient for a long time, it will produce the situation of yawning, and this phenomenon belongs to a normal physiological reaction. 3. Metabolic encephalopathy: it is related to diabetes, liver disease and chronic renal insufficiency, etc. These systemic diseases lead to the damage and confusion of the brain function, and even the function of the brain cells will be damaged due to metabolic diseases, so there may be the symptom of yawning all the time. If the patient has other symptoms besides yawning, he must go to the hospital for examination and treatment in time.