The main reasons for dry mouth while sleeping are as follows: 1, diabetes: can be diagnosed by blood sugar testing, if it is found that fasting blood sugar or two hours after meal blood sugar appears to be elevated, then it is necessary to add an oral glucose tolerance test, and at the same time do glycated hemoglobin or islet function testing to help diagnose whether diabetes is possible. Dry mouth, excessive drinking, and polyuria are typical symptoms of diabetes, so dry mouth will occur at bedtime; 2. Urogyria: Patients with urogyria usually have a decrease in urine specific gravity, drink more water, and also have significant dry mouth at night, requiring water; 3. Hyperparathyroidism: Mostly hypercalcemia leads to thirst, so calcium and phosphorus in the blood and PTH, or parathyroid hormone tests, can be done to help Diagnose whether there is the possibility of hyperparathyroidism; 4. Hyperthyroidism: thyroid function test and thyroid ultrasound can be done to help diagnose whether hyperthyroidism occurs; 5. Dry syndrome: it is usually rare and belongs to rheumatic immune diseases. Because of hypothyroidism and always feel dry mouth, need to drink more water. At this time, you can go to the rheumatology and immunology department to do the diagnostic test related to dry syndrome.