To most, yawning is a sign of sleepiness. But a new study shows that it could also be the body's way of cooling down the brain.

Researchers at Princeton University and the University of Arizona found that people are more likely to yawn in the wintertime, since the temperature of the outside air is cooler than their internal body temp. That's because when you yawn, you bring outside air into your body -- and when you yawn to bring in cold air, it cools down your brain.

From Princeton University:

The researchers concluded that warmer temperatures provide no relief for overheated brains, which, according to the thermoregulatory theory of yawning, stay cool via a heat exchange with the air drawn in during a yawn.

Researchers also found that people are also less likely to yawn when the outside temperature is either equal to or higher than internal body temperature. The study was based on 160 people in Arizona, who were found to yawn twice as often in the wintertime as they do in the summertime.

The yawning finding could also help to explain why people feel more tired when they are warm, The Telegraph reported. Body temperatures are highest when a person is about to sleep.

The finding was published recently in the journal Frontiers in Evolutionary Neuroscience.

Previously, study researcher Andrew Gallup, of Princeton University, had found the same phenomenon in parakeets, Discovery News reported.

"Brains are like computers," Gallup, previously of Binghamton University, told Discovery News. "They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain."