It is generally not normal for a 9-year-old child to sleep with his or her mouth open. Under normal circumstances, a 9-year-old should sleep with his or her mouth closed, and changes in sleeping position should not lead to the phenomenon of sleeping with the mouth open. 9-year-olds sleeping with their mouths open can affect the development of the mouth, teeth, and face, leading to receding jaws, protruding upper jaws, crowded and uneven teeth, dry mouth, and reduced sleep quality, and in some cases, children with snoring symptoms. When a 9-year-old child sleeps with his or her mouth open, parents should take the child to a regular hospital’s ENT department for an examination to determine if the child has rhinitis, deviated nasal septum, polyps, tumors, enlarged adenoids, enlarged tonsils and other diseases that cause poor breathing. Because of poor breathing, the mouth is instinctively used to compensate for breathing to ensure that enough air can be inhaled. There are also children who breathe well during the day and do not need to open their mouths to breathe, but when they lie down at night, their noses do not work and they sleep with their mouths open. Oral developmental problems can also cause 9-year-olds to habitually sleep with their mouths open, even when they are not breathing through them. If all is well with the 9-year-old child’s breathing, a visit to the hospital’s dental department is recommended to get the child to stop sleeping with his or her mouth open with a custom-fitted orthodontic device.