Mucus in the mouth when waking up in the morning may be caused by respiratory system diseases, digestive system diseases, inflammation in the mouth and other diseases, as follows. 1. Respiratory diseases: acute bronchitis, chronic bronchitis, pneumonia, bronchial asthma, pharyngitis and other respiratory diseases may appear mucus sputum, resulting in mucus in the mouth when waking up in the morning. 2. Digestive system diseases: esophagitis, gastritis, gastric ulcer and other digestive system diseases may appear gastric acid reflux to the mouth, resulting in increased mucus in the mouth. 3. Inflammatory diseases in the mouth: oral ulcers, papillitis, periodontitis and other diseases have inflammatory exudate, mixed with saliva, resulting in increased mucus in the mouth. Morning oral mucus may have other reasons, it is recommended to go to the hospital in a timely manner, after a clear cause for targeted treatment.