The possible causes of dry mouth at night sleep are chronic obstructive parotitis, head and neck tumor radiotherapy, dry syndrome, etc., as follows. 1. Chronic obstructive parotitis: it is often caused by narrowing of the duct system, blockage of salivary gland duct stones or foreign objects, which obstructs the flow of saliva and leads to dry mouth, and squeezing the parotid gland can lead to the flow of viscous egg-white-like saliva from the mouth of the duct. 2. Head and neck tumor radiotherapy: it may lead to the damage of salivary gland’s alveolar cells, and the decrease of saliva secretion leads to dry mouth. 3. Dry syndrome: the destruction and disappearance of salivary gland vesicle cells, salivary secretion is significantly reduced, causing dry mouth. There may be other reasons for dry mouth at night, it is recommended to go to the hospital in time, with the help of the doctor to clarify the cause of the disease after targeted treatment.