1. Oral disease: Poor hygiene oral cavity is the most direct cause of bitter taste in the mouth, especially for those who don’t like to brush their teeth after dinner or before going to bed. The food residues in the mouth will continue to breed during sleeping time. Oral diseases such as gum inflammation or bleeding gums can also cause this condition.

2. Digestive related disease: Irregular diet, overeating or excessively irritating food (such as spicy food) can also easily lead to indigestion in the stomach and damage the mucosa of the stomach lining wall. When the secretion of gastric acid is abnormal, the digestive function will be disrupted, and gastroesophageal reflux may even occur during sleeping at night.

3. Gallbladder disease: The bile stored in the gallbladder tastes bitter, the bile involved in breaking down fat needs to flow through the duodenum. If gallbladder diseases such as cholecystitis and gallstones occur, the bile excretion or secretion will be abnormal. Once it is reflux to the esophagus with gastric juice, it will appear bitterness in the mouth.

4. Heaty liver: From the perspective of traditional Chinese medicine, if a person is stressed, it will result in heaty liver and often feel irritable. The liver qi stagnates and liver heat is created, which brings bitter taste to the mouth.