QUESTION: I have often wondered just exactly what it is that triggers the yawn.
– James Leach,
ANSWER: According to folk belief, we yawn because we are not breathing in enough oxygen. The deep inhalation that is a major feature of the yawn makes this idea appealing, but there is compelling evidence against it.
When people were made to breathe air with higher than normal levels of carbon dioxide, their respiration rate increased, but they did not yawn more than people breathing normal air. The number of yawns also did not change when people breathed pure oxygen. Respiration rate, rather than yawning, seems to regulate oxygen intake.
So why do we yawn? One possibility is that yawning stimulates us to stay awake. Studies have shown that people yawn frequently in the hour before they go to bed, but rarely yawn when they are trying to fall asleep. People also yawn frequently while driving. Zoo and laboratory animals yawn before their normal feeding time. Yawning seems to occur when it is important to stay awake.
Some scientists think yawning may dilate the arteries that bring blood to the brain, thereby increasing cerebral blood flow.
The exact trigger remains elusive. Certain research suggests that an oxygen sensor, located in the hypothalamus, initiates the yawn in response to low levels of oxygen in the brain. Since blood carries oxygen, this research is consistent with the idea that yawning causes a jump in blood flow to the brain.
Although nearly all vertebrates yawn, until recently, humans were the only species known to yawn contagiously. However, recent research has shown that chimps yawned more when watching a video of other chimps yawning.
Sherry Seethaler has a doctorate in science education and is a UCSD science writer and educator.