There are many causes for overbite, crooked teeth, and crowded teeth. Patrick McKeown from The Buteyko Clinic believes that mouth-breathing is a direct cause of oral disfigurement. Reason being, the tongue puts an immense amount of pressure on the teeth when air is inhaled and exhaled from the oral cavity. This pressure is significant enough to move teeth, change your facial structure, and obstruct your airways. When mouth-breathing becomes a habit during the day, it also leads to mouth breathing at night: this is a known cause of snoring and sleep apnea.