A yawn is a common and often involuntary reflex observed across many vertebrate species, from fish to humans. This universal action involves a distinctive wide opening of the mouth, accompanied by a deep inhalation and a slow exhalation. While seemingly simple, the precise reasons behind this pronounced mouth gaping have long intrigued scientists. Understanding why we open our mouths so widely during a yawn involves exploring its physiological events and proposed functions.
The Physical Act of Yawning
The physiological sequence of a yawn begins with a gradual, deep inhalation of air, often through both the nose and mouth. This inhalation is accompanied by a significant opening of the lower jaw, creating a wide gape. As the jaw extends, various muscles in the face, neck, and chest stretch in coordination.
Muscles such as the diaphragm and intercostal muscles in the ribs contract, facilitating the deep breath. Simultaneously, muscles around the airway, including subhyoidal muscles and those in the floor of the mouth, contract, causing the pharynx to dilate considerably. The powerful stretching involves jaw muscles like the masseter and temporal, along with pharyngeal muscles.
At the peak of the yawn, there is a brief moment of maximal muscle tension and stretching. This peak can also involve an instinctive stretching of other body parts, such as the arms, and may cause the eyes to close tightly, sometimes leading to watering. Following this peak, the muscles relax, and a slow exhalation occurs, often accompanied by a sense of relief or satisfaction.
The Air Exchange and Muscle Stretch Theory
One long-standing hypothesis suggested that the wide mouth opening during a yawn serves to improve respiratory gas exchange, specifically to increase oxygen or decrease carbon dioxide in the blood. This “respiratory hypothesis” has largely been set aside by research. Studies have demonstrated that altering oxygen or carbon dioxide levels in the air people breathe does not significantly affect yawning frequency.
Instead, another aspect of this theory focuses on the physical stretching that the wide mouth opening enables. The expansive gape stretches numerous muscles in the jaw, face, and throat. This muscular extension may help alleviate stiffness and improve the range of motion in these areas.
Stretching of the muscles involved in chewing, swallowing, and vocalization could contribute to their efficient function. The act of stretching, which often accompanies a yawn, is also linked to an arousal response, preparing the body for activity or increasing alertness. The wide mouth opening is integral to these stretching actions, potentially offering a physical preparation or restorative function for the associated muscles and joints.
The Brain Cooling Theory
A prominent theory suggests that the wide mouth opening during a yawn functions as a mechanism to regulate brain temperature. This “brain cooling hypothesis” posits that yawning is triggered when the brain’s temperature slightly increases, often due to fatigue, drowsiness, or stress. The deep inhalation of cooler ambient air through the widely opened mouth directly cools the blood flowing through the oral and nasal cavities.
Simultaneously, the stretching of facial muscles during the yawn enhances blood flow to the head and neck. This increased circulation aids in dissipating heat from the brain, as warmer blood is moved away and cooler blood is brought in. The cooled blood then circulates to the brain, helping to lower its overall temperature and maintain optimal cognitive function.
Research supports this idea, showing that yawning frequency can increase with brain temperature and that yawns are followed by a decrease in brain temperature. Studies have also observed that yawning is more frequent within a specific “thermal window” of ambient temperatures, becoming less common when outside temperatures are either too cold or too hot to provide effective cooling. The pronounced mouth opening is a direct and active component of this brain temperature regulation.