The Yawn's Thermal Window

The most direct test of the oxygen hypothesis came in 1987, when psychologist Robert Provine, together with Bruce Tate and Leonard Geldmacher, had students breathe air mixtures enriched with carbon dioxide or with pure oxygen. Neither the number of yawns nor their duration changed — and physical exercise strenuous enough to double the breathing rate did not move the result either. The oxygen hypothesis left the conversation on data, not assumptions.

Data from two distinct field studies support a link with temperature without settling yawning's ultimate function on their own. Drag the slider below and trace the measured shape of contagious yawning in two cities, a continent apart.

A thermal window, measured in two cities

In 2011, researchers Andrew Gallup and Omar Eldakar stopped 160 pedestrians on the streets of Tucson, Arizona, across two seasons, and showed them photographs of people yawning. In winter, at 22°C, 45% of those stopped (36 of 80) yawned back. In summer, at 37°C — close to body temperature — only 24% (19 of 80) did. Temperature remained the only factor that statistically explained the difference, even after the researchers separately checked for the previous night's sleep, time spent outside, and humidity.

Three years later, a team led by Jorg Massen repeated the idea in Vienna, with 120 pedestrians, to test the cold end of the hypothesis as well. At 1.4°C, only 18.3% (11 of 60) yawned contagiously; at 19.4°C, close to the temperature where Tucson had recorded its peak, the rate rose to 41.7% (25 of 60). The two cities used different teams, years, and populations, so the figures are never added into a single statistic — but read side by side, they trace the same shape: a peak somewhere around 20°C and a decline toward both extremes, cold and hot alike. Within each city on its own, the peak rate runs close to twice the rate at the tested extreme: 1.9 times in Tucson, 2.3 times in Vienna.

The proof from the lab

If outdoor temperature merely correlates with something happening inside the head, the next step is to change that something directly. In a 2007 experiment, Andrew Gallup and Gordon Gallup Jr. showed volunteers videos of people yawning while controlling either their breathing route or their forehead temperature. The results were consistent with the direction predicted by the brain-cooling hypothesis.

Nasal breathing, which pre-warms and humidifies the incoming air more before it reaches the base of the brain, cut contagious yawning from 48% to 0%. A cold pack held to the forehead — a completely different route to cooling the same region — cut it from 41% to 9%. In both cases, the body already had access to a cooling route; it no longer needed to open the "window" through a yawn.

The mechanism behind the window

The researchers' proposed mechanism ties both sets of results to a single model. The deep inhalation of a yawn stretches the jaw, raises pressure inside the chest, and sharply increases venous return from the head. That venous blood, cooled by air drawn in through the nose and mouth, could reach the cavernous sinus — a small pool around the internal carotid artery, right at the base of the skull — and draw heat away from the arterial blood about to enter the brain. In effect, a short, fast radiator. This model would explain why it only works within a temperature window: if the outside air is hotter than the body, a deep inhalation cools nothing; if the outside cold is already strong enough that the body is cooling anyway through other routes, the pressure to open the window drops too.