Christoph Niemann

THE FACTS

The medical literature is rife with explanations for yawning, but one has gained substantial ground in recent years: This mysterious habit may help regulate brain temperature.

The brain operates best within a narrow range of temperatures, and like a car engine, it sometimes needs a way to cool down. To lower the brain’s thermostat, researchers say, the body takes in cooler air from its surroundings — prompting deep inhalation.

Yawning is contagious. Simply watching someone do it is enough to induce the behavior. But when scientists had people watch yawning videos in a 2007 study, they found that applying cold packs to the subjects’ heads practically eliminated contagious yawning. Nasal breathing, which also promotes brain cooling, had a similar effect.

In a study of 160 people published last month in the journal Frontiers in Evolutionary Neuroscience, yawning was found to vary by season. People were shown to be more likely to yawn in winter than summer, perhaps because an overheated brain gets little relief from taking in air that is warmer than body temperature.

The researchers, who controlled for factors like humidity and the amount of sleep subjects got the night before, also found that the more time a person spent outside in warm temperatures, the less likely they were to yawn. The findings may explain why people yawn when tired: Sleep deprivation raises brain temperature. As for why yawning is contagious, it may have evolved as a way to signal to others in a group to stay alert and ready in case of outside attacks, scientists say.

THE BOTTOM LINE

Growing evidence suggests yawning may be a way for the brain to cool off, though it is still just a theory.