Nocturnal mouth breathing, or oronasal breathing, occurs when air is inhaled or exhaled through the mouth instead of the nose during sleep. Relying on the mouth for nighttime respiration indicates an underlying issue, as the nose is specifically designed to filter, warm, and humidify incoming air. Chronic mouth breathing bypasses this conditioning process and can lead to health concerns, including dry mouth, poor sleep quality, and dental issues.
Structural Airway Barriers
One of the most common reasons people breathe through their mouth at night is a physical blockage within the nasal passages that prevents sufficient airflow. These are fixed anatomical issues that physically narrow the pathway, forcing the body to compensate by using the mouth. A deviated nasal septum is a frequent culprit, where the wall dividing the nostrils is significantly shifted to one side, constricting one or both nasal chambers. This structural misalignment can be present from birth or result from an injury.
The size of the tonsils and adenoids, which are lymphatic tissues, can also physically obstruct the upper airway. Located at the back of the throat and behind the nose, these tissues often become chronically enlarged, especially in children, due to repeated infections or inflammation. When these tissues swell, they reduce the available space for air to pass, a problem often worsened when lying down. Nasal polyps, which are non-cancerous, inflammatory growths on the lining of the nasal passages, represent another physical blockage. These growths can obstruct the nasal cavity, making nasal breathing impossible and leading to the switch to mouth breathing.
Inflammation and Temporary Congestion
In contrast to fixed structural barriers, temporary congestion caused by inflammation is a dynamic cause of mouth breathing. The nasal passages contain turbinates, which are structures covered in highly vascularized tissue designed to warm and humidify the air. Allergic rhinitis (hay fever) causes the immune system to release inflammatory chemicals like histamine in response to environmental triggers. This release causes the turbinate tissues to swell dramatically (turbinate hypertrophy), effectively closing the nasal passage.
Similar inflammatory responses occur during an acute illness, such as a common cold or flu, causing significant swelling of the nasal lining. Chronic sinusitis, or long-term inflammation of the sinus cavities, also leads to persistent tissue swelling and mucus accumulation that blocks normal airflow. When a person lies flat to sleep, increased blood flow to the head can intensify this swelling, worsening nasal blockage. This temporary, nightly congestion forces the individual to rely on the mouth to maintain adequate air intake.
Sleep Position and Learned Behavior
Beyond physical and inflammatory blockages, mechanical factors related to sleep posture and learned habits contribute to nocturnal mouth breathing. Sleeping on one’s back (the supine position) allows gravity to pull the lower jaw, tongue, and soft palate backward toward the throat. This movement narrows the upper airway, even in individuals with clear nasal passages, making nasal breathing more challenging. The body’s reflexive response to this restricted airflow is to open the mouth to acquire air more easily.
For some individuals, mouth breathing persists as a learned behavior even after the original cause, such as a childhood cold or allergy, has resolved. The body becomes accustomed to the open-mouth pattern, maintaining the habit throughout the night despite the ability to breathe nasally. This habitual pattern can be reinforced over time, continuing the cycle of dry mouth and disrupted sleep. Correcting sleep posture to side sleeping often minimizes the gravitational effect on the airway, encouraging the return to nasal breathing.
Connection to Sleep Apnea
Nocturnal mouth breathing is often a coping mechanism or a direct symptom of Obstructive Sleep Apnea (OSA). OSA is a disorder where the upper airway repeatedly collapses partially or completely during sleep, causing pauses in breathing. When the airway narrows due to the relaxation of throat muscles, the body instinctively opens the mouth to draw in more air.
This open-mouth breathing serves as a compensatory maneuver to overcome the blockage, but it can worsen the underlying condition. Breathing through the mouth can further destabilize the airway and increase the likelihood of collapse, creating a detrimental feedback loop. Mouth opening and mouth breathing are common in OSA patients and often relate to the severity of the disorder. Because mouth breathing in this context indicates a potential breathing disorder that fragments sleep and reduces oxygen levels, any chronic occurrence should prompt a medical evaluation.