Why Do We Yawn? Contagious Yawns Help Cool Down the Brain, Scientists Say

Scientists who prevented people from catching yawns in a study believe the reflex helps to cool down the brain.

The study, published in the journal Physiology & Behavior, involved 92 undergraduate biology students who changed the temperature of their brains in a lab. The team wanted to test the hypothesis that yawning is triggered when the skull gets too hot, and helps to cool the brain by taking warm blood away while bringing in a cooler supply.

The temperature of the brain can change for a variety of reasons, including stress, cortical arousal and sleep patterns.

Participants were instructed to place a cold (4 C/39.2 F), warm (46 C) or room temperature (22 C) pack on their carotid arteries: major blood vessels in the neck that pump blood towards the brain, neck and face. They held the compress in this area for five minutes, and scientists used thermographic imaging equipment to check if the temperature of participants' brain was altered. It's not possible to accurately test the temperature of the brain without an invasive procedure.

Next, the participants watched a 63-second-long video of nine different people yawning. After watching the clips, they filled out questionnaires answering whether they had the urge to yawn before, during or since. As yawning is contagious, scientists thought this would test whether the temperature of the blood heading to their brain affected this reflex.

As expected, the scientists found cooling was linked to fewer yawn urges. Of the total, 62 participants felt the need to yawn while watching the video. Participants who cooled their brain temperature had less of an urge to yawn than both those in the warm and room temperature groups. A total of 48.5 percent of participants in the cool group felt like yawning, compared with 84.8 percent of those in the warm group; and 69.2 percent in the room temperature group.

"These findings are consistent with previous research indicating that yawns function as a compensatory brain cooling mechanism," the authors wrote.

However, the scientists didn't notice a statistically significant difference between yawning in the warm and room temperature groups. The authors argued this could be because the heating pack may have created a temperature too high for yawning to ease.

Andrew C. Gallup, study co-author and assistant professor of psychology at SUNY Polytechnic Institute, told PsyPost: "Yawning is often misunderstood both within the scientific community and the general public."

Gallup and his team showed in a previous study that people are less likely to catch yawns during the winter than in the summer months. That 2014 work was also published in the journal Physiology & Behavior.

Other theories as to why we yawn include one posited by researcher Olivier Walusinski in the journal Clinical Anatomy in 2013. He argued the reflex helps to send cerebrospinal fluid around the brain.