11 July 2014


Welcome back. Are you as curious as I am about why we yawn? I know when we yawn--we’re sleepy, bored, stressed; but why do we yawn? It’s a real head-scratcher because the winning answer should explain yawning by you and me as well as by unborn babies not to mention most vertebrate animals.

Joseph Ducreux Self Portrait Yawning; Google_Art_Project
My regular source of ever-changing health facts, the Berkeley Wellness Letter, let me down on this one, stating that the cause is unknown. They didn’t even mention the one theory that seems to fit all cases: brain cooling. Yep, you read that right. Go ahead. Giggle! Jeer! Here’s the story.

Thermoregulatory Theory

I won’t take the time to dispense with other theories of why we yawn (e.g., lack of oxygen). None is supported by the research. In contrast, brain cooling or more properly thermoregulation has been posed and experimentally refined--especially by one investigator who’s currently at the State University of New York (SUNY) at Oneonta--and is yet to be disproven.

The general idea is that yawning keeps the temperature of the brain in equilibrium. The stretching of the jaw, gaping of the mouth, deep inhalation followed by shallow exhalation increases blood flow in the area of the head; the air intake forces the flow of blood and spinal fluid from the brain; and the air breathed into the mouth cools these fluids.

Think of a radiator that exchanges cooler blood from the lungs and extremities for warmer blood from the brain. You’ve heard that cooler heads prevail? Well, this is cooler brains function more efficiently.

Ambient Air Temperatures

If you entertain the theory, you realize that the temperature of ambient (external) air would have a major affect on yawning. The frequency of yawning would be expected to:
- increase as air temperature rises and stimulates the thermoregulatory mechanisms,
- decrease if air temperatures approach or exceed body temperature (no cooling effect), and
- decrease if air temperatures fall below the point where cooling is needed.

In short, the theory suggests there should be an optimal range of air temperatures and this lends itself to testing.

Experimental Findings

In an early test of the theory by investigators at SUNY Albany, participants were shown videotapes of people yawning. (There’s no question about yawning being contagious.) Participants who held a hot pack (115 F or room temperature) to their forehead while watching the videos yawned 41% of the time. Those who held a cold pack (39 F) to their forehead yawned only 9% of the time.

A subsequent experiment conducted in Tucson, Arizona, by researchers from Binghamton University and the University of Arizona, asked 160 pedestrians to look at photographs of people yawning. Half of the participants were queried in winter (about 72 F) and half in early summer (nearly 99 F). In winter, 45% of the people yawned, while in summer, only 24% of the people yawned.

In a recent collaborative effort, investigators from the University of Vienna, Nova Southeastern University, Ft. Lauderdale, and SUNY Oneonta repeated the Arizona experiment in Vienna, Austria, with 120 pedestrians (60 in summer, 67 F; 60 in winter, about 34 F). Yawning frequency was the reverse of that found in Arizona: approximately 42% in summer and 18% in winter.

Merging the Vienna and Arizona results, the optimal air temperatures for contagious yawning centered around 68 F, tailing off as air temperatures near body temperature or freezing.

Wrap Up

I’d like to think that students who yawned in my classes--a very rare event--were just cooling their brain so they could squeeze in more information. Thanks for stopping by. I hope you feel it was worth your time.


Selected studies on brain cooling in various journals:
2007 Evolutionary Psychology:
2008 Frontiers in Physiology & Behavior:
2010 Frontiers in Evolutionary Neuroscience:
2011 Frontiers in Evolutionary Neuroscience: journal.frontiersin.org/Journal/10.3389/fnevo.2011.00003/full
2011 Medical Hypotheses:
2013 Frontiers in Neuroscience:
2014 Physiology & Behavior: www.sciencedirect.com/science/article/pii/S0031938414001784

Science Daily article on 2014 paper:
UC Berkeley Wellness Letter article on yawning:
LiveScience article on fetus yawning:
WebMD article on yawning:

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