A typical yawn can stretch the jaw nearly 2 inches wide and engages almost every muscle in the face.

Yawning is a complex physiological event known as an involuntary respiratory reflex. During a yawn, the jaw can reach an aperture of 35 to 50 millimeters, which is significantly wider than the 10 to 20 millimeters typically used during speech. This action involves the simultaneous inhalation of air and the stretching of the eardrums, followed by a shorter exhalation.A leading theory, proposed by Dr. Andrew Gallup of SUNY Polytechnic Institute in 2007, suggests that yawning serves as a brain-cooling mechanism. The deep inhalation brings in cooler air, while the intense jaw stretch increases blood flow to the brain, allowing heat to dissipate more efficiently. This thermoregulation helps maintain optimal mental performance and alertness when the body is tired or bored.A 2016 study published in the journal 'Biology Letters' analyzed 29 different mammal species to find a link between yawn duration and brain weight. The researchers discovered that species with larger brains and more cortical neurons, such as humans and elephants, have significantly longer yawns than smaller-brained animals like mice or rabbits. This suggests that the physical act of yawning is scaled to the metabolic needs of the brain's complexity.Furthermore, yawning engages approximately 20 different muscles, including the diaphragm, intercostals, and various facial muscles. This widespread muscular activation helps stimulate the carotid artery, which may further trigger a state of heightened physiological arousal. While often associated with sleepiness, yawning actually serves as a biological reset to keep the brain functioning at its peak.