The bitterness of mouth when sleeping may be due to oral diseases such as dental caries, functional dyspepsia and digestive system diseases, the specific reasons are as follows: 1. Dental caries and oral related diseases, dental caries is easy to retain food residues and bacteria, fermentation will produce bitterness in the mouth when sleeping, it is necessary to deal with dental caries, if necessary, the dental caries should be extracted. Periodontitis, pulpitis, mouth ulcer and other oral diseases will also cause such symptoms, need to treat the disease symptomatically. 2, functional indigestion, the spleen and stomach digestive function is reduced, can not digest and absorb the diet in time, stay in the stomach and intestines will feel bitter mouth, pay attention to the diet, eat more vegetables, fruits. Pay attention to exercise in general to promote gastrointestinal peristalsis and improve the symptoms of indigestion and constipation. 3, digestive system diseases, such as gastritis, gastric ulcer, duodenal ulcer patients often have symptoms of bitter mouth, need to take drugs under the guidance of doctors, and actively cooperate with treatment.