Yawning can be considered as sign of many things, most notably sleepiness and boredom. But new science is adding another option to the list: hot-headedness.
A recent study out of Princeton University's Department of Ecology and Evolutionary Biology suggests yawning could serve the purpose of cooling our brain. The study, Science Daily reports, is the first involving humans associating variability in yawning frequency with the season. It found that people are less likely to yawn when the temperature outside exceeds body temperature -- so, hotter than the 98.6.
To come to this conclusion, study co-authors Andrew Gallup from Princeton and Omar Eldakar from the University of Arizona's Center for Insect Science reviewed the yawning frequency of 160 subjects in Tucson, Arizona, in the winter and summer -- 80 people were documented each season. They found more people yawning in the winter, which supports the thermoregulatory theory of yawning. Science Daily has Gallup's explanation of the findings:
"According to the brain cooling hypothesis, it is the temperature of the ambient air that gives a yawn its utility. Thus yawning should be counterproductive -- and therefore suppressed -- in ambient temperatures at or exceeding body temperature because taking a deep inhalation of air would not promote cooling. In other words, there should be a 'thermal window' or a relatively narrow range of ambient temperatures in which to expect highest rates of yawning.
"Our study accordingly showed a higher incidence of yawning across seasons when ambient temperatures were lower, even after statistically controlling for other features such as humidity, time spent outside and the amount of sleep the night before. Nearly half of the people in the winter session yawned, as opposed to less than a quarter of summer participants.
These results provide additional support for the view that excessive yawning may be used as a diagnostic tool for identifying instances of diminished thermoregulation."
But there are some other common theories as the the function of the yawn. The most well-known is that it is delivering a rush of oxygen to the brain when you're tired. Discovery Health reports Robert Provine, a development neuroscientist at the University of Maryland, Baltimore County, as noting that while yawning is often associated with tiredness, heart rate rise as much as 30 percent during a yawn. Some theorize that yawning is a vestigial function leftover from our ancestors, others that it happens due to a build up of carbon dioxide, and others hold to the boredom theory.
And what about a contagious yawn? According to Discovery Health, one reason we yawn in part to get rid of a build up of carbon dioxide. Which is why there is a theory that in groups, we are induced to yawn -- potentially making it look contagious -- because of a build-up of exhaled carbon dioxide.
Watch this MythBusters clip to see if contagious yawns are fact or fiction:
Even though the Myth Busters suggest a larger sample size, which would increase carbon dioxide in the room, Provine tested this theory and found that more oxygen in a room didn't decrease yawning, nor did decreasing carbon dioxide.
This story has been updated for clarity.