Yawn

A yawn is an involuntary act characterized by a wide opening of the mouth, followed by a deep inhalation, a brief hold, and a slower exhalation. It is often accompanied by stretching of the jaw and sometimes the limbs. Yawning is produced by neural circuits in the brainstem and can be triggered by various internal states such as tiredness or boredom.

Common triggers include sleepiness, fatigue, boredom, and low energy. The function of yawning is debated; proposed roles include increasing arousal, regulating brain temperature through increased blood flow and inhalation, and extending social communication. The popular oxygen-uptake theory has fallen out of favor, but the thermoregulatory and brain-ventilation hypotheses remain discussed.

Contagious yawning refers to yawning induced by others' yawns or even hearing about yawning. It is well documented in humans and some other species and appears to be linked to social bonding and empathy in some individuals, though not everyone is susceptible. Contagiousness can be reduced by certain factors such as fatigue, medications, or neurological conditions.

Yawning is common across many vertebrates, including humans and other mammals; it has also been observed in some birds. It may serve multiple functions depending on context, from signaling wakefulness to coordinating group behavior. If yawning is frequent and unexplained, it can be associated with fatigue, stress, or medical conditions and may warrant medical evaluation.