Sleeping with open mouth breathing may be caused by adenoid hypertrophy, rhinitis, deviated nasal septum and so on. 1. Adenoid hypertrophy: the adenoids are pathologically enlarged, which can block the airway and lead to open-mouth breathing during sleep. 2. Rhinitis: including acute rhinitis or chronic rhinitis, due to the inflammatory reaction leading to congestion and swelling of the nasal mucosa, causing nasal congestion, poor breathing, and breathing through the mouth when sleeping. 3. Deviated nasal septum: When the nasal septum is severely deviated, it often leads to the nasal cavity is not smooth, and it can cause nasal cavity dysfunction, and the symptoms of open-mouth breathing and snoring may appear when you sleep. Sleeping with open mouth breathing may also be caused by other reasons, it is recommended to go to the hospital in time to find out the cause of the relevant treatment.