The Difference Between an Open Mouth and Actual Mouth Breathing

This study measured nasal and oral airflow simultaneously overnight in 85 adults, using sensors placed just in front of the lips as well as the nose. It found that oral airflow during sleep isn't a single phenomenon: some episodes cluster around apnoeas and hypopnoeas, while a separate pattern begins during otherwise stable breathing and is only loosely tied to respiratory events. This second pattern correlated specifically with nasal obstruction, suggesting it functions as a workaround, air finding a way out when the nose is too congested to manage it alone.

The distinction matters for anyone whose partner notices their mouth is open at night: an open mouth is not automatically evidence of disordered breathing, and this research helps separate a nasal bypass mechanism from more troubling breathing patterns. The main limitation is that participants were drawn from a sleep clinic population being assessed for suspected sleep-disordered breathing, so the findings may not generalise cleanly to people with no breathing concerns at all.