Sleeping with the mouth open, the common clinical causes are adenoid hypertrophy, nasal polyps, improper sleeping posture, rhinitis, tonsil hypertrophy, obesity and so on. 1. Adenoid hypertrophy: If there is adenoid hypertrophy at the junction of pharynx and end of nose, it will cause narrowing of the airway and affect the ventilation, which will lead to open-mouth breathing, snoring and nasal congestion and other symptoms. 2. Nasal polyps: the pathogenesis of this disease is not clear, usually associated with genetics, pathogenic microbial infections, immune mechanisms and anatomical abnormalities and other factors, the clinical manifestations of ear stuffiness, open-mouth breathing during sleep, nasal congestion, runny nose, loss of sense of smell and so on. 3. Improper sleeping position: this is a physiological reason. Part of the population sleeps flat on their backs, and their heads will be tilted backward, and the maxillary tissues will be tilted backward due to gravity, which will cause the mouth to be unable to close, thus leading to open-mouth breathing. 4. Rhinitis: People suffering from this disease do not keep their nasal passages well ventilated when they sleep, so they will unconsciously change to open-mouth breathing. 5. Tonsillar hypertrophy: this is a common disease in children, which is usually associated with inflammation of local or neighboring parts, long-term stimulation of nasal purulent secretions, etc. Clinical manifestations include nasal congestion, open-mouth breathing, snoring and so on. 6. Obesity: The adenoids or tonsils of obese people may be enlarged, thus the nasal breathing resistance of these people increases and they turn to open-mouth breathing. Those who are affected in their daily life are recommended to seek medical advice for a clear diagnosis and treatment as prescribed by the doctor.