The brain cooling theory of yawning has gained substantial scientific support in recent years. This hypothesis posits that yawning serves as a thermoregulatory mechanism to cool the brain through increased blood flow and heat exchange with inhaled air.1 Studies have demonstrated that yawning frequency is influenced by ambient temperature, with an optimal "thermal window" for yawning around 20°C.2 Yawning decreases at extreme temperatures, both hot and cold, further supporting its thermoregulatory function.3 Researchers have found that the duration of yawns correlates positively with brain size and neuron count across mammalian species, suggesting that animals with larger and more complex brains require longer yawns to achieve the same cooling effect.4 This evidence, combined with observed physiological changes during yawning such as increased blood flow to the head and face, provides strong support for the brain cooling theory as the primary explanation for this ubiquitous behavior.1

The physical act of yawning triggers significant circulatory changes in the body, supporting its potential role in brain cooling. Studies have shown that yawning causes a marked increase in heart rate, with one investigation finding a significant rise at the peak of yawning and for several seconds afterward compared to baseline levels1. This acceleration in heart rate is accompanied by increased blood flow to the head and face regions. The contraction and relaxation of facial muscles during a yawn enhance facial blood flow, potentially aiding in heat dissipation through emissary veins1. Additionally, yawning leads to an increase in skin conductance, indicating heightened arousal similar to that induced by stimulants like caffeine1. These physiological responses collectively suggest that yawning may facilitate the removal of warm blood from the brain while introducing cooler blood, thus contributing to temperature regulation in the central nervous system21.

Contagious yawning has long been associated with empathy, but recent research suggests this link may be more complex. While some studies have found a correlation between empathy and susceptibility to contagious yawning, others challenge this connection. A study published in i-Perception found that perceptual sensitivity, rather than empathy, was more closely related to contagious yawning in non-clinical populations1. However, the phenomenon does appear to have social aspects. Auditory contagious yawning (triggered by hearing but not seeing a yawn) is highest between friends and family members, suggesting a social bonding component2. Yawning may also serve as a non-verbal communication tool, potentially signaling tiredness or boredom and helping to synchronize group behavior3. Despite these social theories, the exact relationship between yawning, empathy, and prosocial behavior remains unclear and requires further research4.