Infants sleeping with their mouths open may be a normal physiological phenomenon, but it may also be abnormal, mostly related to allergic rhinitis, respiratory tract infections and other reasons, as follows: 1. Physiological phenomenon: babies sleeping with their mouths open may be related to the sleeping posture or more nasal secretions, which is a normal physiological phenomenon. 2. Allergic rhinitis: when the nasal cavity is not clear, the airflow can not enter from the nasal cavity, the baby will have the phenomenon of sleeping with the mouth open, which may also be accompanied by adenoid hypertrophy, tonsillar hypertrophy, teeth grinding, sleep disturbances and other phenomena. 3. Respiratory tract infections: nasal secretions often increase, causing nasal congestion and other symptoms, also easily lead to the phenomenon of sleeping with mouth open. There may be other reasons for babies to sleep with their mouths open, and it is recommended to go to the hospital in time to avoid delaying the condition.