Waking up at four in the morning with a bitter taste in your mouth can be caused by a dry sleep environment, diabetes, dry syndrome, hyperthyroidism, and uremia. But “four o’clock in the morning” and wake up after sleeping at night is not a big difference. 1. Dry sleeping environment: poor ventilation in the bedroom, high temperature, bedding and clothing is too thick and other factors lead to heat and sweat during sleep, the body water loss, so this symptom occurs. 2. Diabetes mellitus: elevated blood glucose levels, glucose into the urine, resulting in increased urine osmolality, a large amount of water into the urine, resulting in water loss, so this symptom will appear. 3. Dry syndrome: autoimmune system mistakenly attacks the salivary glands, resulting in salivary synthesis and secretion disorders, and the oral cavity is too dry and the symptoms appear. 4. Hyperthyroidism: hyperthyroidism, secretion of excessive thyroid hormones, resulting in hypermetabolism of the body tissues, increased heat production, sweating increased accordingly, the body is dehydrated and the symptoms. 5. Uremia: antidiuretic hormone synthesis and secretion disorders, decreased sensitivity of the kidneys to antidiuretic hormone and other factors leading to an increase in urine synthesis, so the symptoms. Waking up at four in the morning with a bitter taste in the mouth may be related to a variety of factors, it is recommended to consult a doctor for a clear diagnosis.