Yawns

Yawns are an involuntary behavior characterized by a wide opening of the mouth, a deep inhalation, and a slower exhalation, often followed by a series of breathing cycles. Yawning occurs in humans and many animals, and is commonly observed during fatigue, boredom, or transitions between wakefulness and sleep, though it can occur in other contexts as well.

Physiology and triggers: The act engages the jaw muscles, facial muscles, and the respiratory system. The precise brain mechanisms are not fully understood, but brainstem circuits, hypothalamic regions, and neural networks involved in arousal and temperature regulation are implicated. Some researchers propose that yawning helps with brain temperature regulation by promoting cerebral cooling through increased blood flow and evaporative cooling from extended inhalation. Yawning frequency tends to rise with tiredness and with cooler ambient brain temperatures.

Contagious yawning: Yawning can be contagious—seeing, hearing, or even thinking about yawning can trigger yawns in others. Contagion is observed in many human groups and in several mammalian species, and is thought to involve the mirror neuron system and areas related to social cognition and empathy. Susceptibility to contagious yawning is influenced by age, fatigue, and social factors; sleep deprivation can increase contagiousness.

Clinical notes and evolution: Yawning is usually benign, but excessive yawning can be associated with sleep disorders, fatigue, or certain medications. Evolutionarily, yawning is found across vertebrates and may serve both physiological and social signaling functions, including signaling vigilance shifts and promoting social bonding in group-living species.