What’s in a yawn? Scientists have many theories about the origin of this common phenomenon, but there’s no clear single explanation for why yawning exists.

The fancier term for stretching and yawning is “pandiculation.” There are lots of things known to make you pandiculate -- boredom, sleepiness, stress. Just seeing someone else yawn can get you going as well.

One early theory suggested that yawning was a way for a person to simultaneously expel a lot of carbon dioxide and gulp down a lot of oxygen if they were breathing too slowly. But experiments in the 1980s showed that neither breathing 100 percent oxygen nor breathing air with higher concentrations of carbon dioxide caused subjects to yawn more.

A recent review of yawning literature in the journal Frontiers in Neuroscience from Princeton University biologist Andrew Gallup and University of Arizona insect biologist Omar Eldakar described how they examined the effects of temperature on yawning by surveying pedestrians in Tucson, Ariz. The pedestrians were quizzed on their yawning behavior and also shown photos of other people yawning.

The researchers found that subjects in winter, which in Arizona corresponds to about room temperature, “were nearly twice as likely to yawn during the survey and that even when accounting for variables such as the amount of sleep the night before, duration of time spent outside prior to being surveyed and relative humidity, ambient temperature was the only significant predictor of yawns.”

Gallup has also done experiments where people held cold packs or warm packs to their foreheads while watching videotapes of people yawning. With a hot pack held to the forehead, the contagious yawning rate was about 41 percent. But when a cold pack was applied, the rate was just 9 percent.

These findings lend some weight to the “brain thermoregulation” theory of yawning, which proposes that yawning helps keep the brain cool. Like a computer, a mammalian brain can’t function if it overheats.

There are a few different proposed mechanisms for how a yawn could lower brain temperature, according to Gallup and Eldakar. Yawning accelerates the heart rate, elevates blood pressure and increases blood flow in the neck, head and face, in particular. Bringing in a bunch of cooler blood from the lungs and extremities to carry off heat could help lower the brain’s temperature the way that a car’s radiator helps keep the engine cool.

Yawning could also cool the brain more directly thanks to the deep inhalation of cooler ambient air or because the extreme jaw-stretching motion helps ventilate the sinuses.

The contagiousness of yawns suggests there may be a deeper social context for pandiculation as well. You’re more likely to catch a yawn from someone you’re close to than a stranger -- a pattern that has been observed in bonobos as well as humans. People that have empathy-related disorders, like autism or schizophrenia, are less likely to find yawns contagious.

There’s also a surprisingly erotic side to yawning as well.

“In several proverbs and sayings, yawning -- and especially contagious yawning -- is interpreted as a clue of something more than just sympathy, that is, as a sign of being in love,” researcher Wolter Seuntjens wrote in his 2004 dissertation.

Giving certain drugs to rats -- clavulanic acid, pramipexole -- has been shown to induce both yawning and erections, suggesting there’s some dopamine-activated pathway in the brain that can trigger multiple effects. Animals dying of rabies have been found to show signs of both yawning and spontaneous ejaculation, according to Seuntjens.

Therefore, don’t get too bent out of shape if your significant other pandiculates in intimate moments. Since pandiculation and sexual climax seem to share certain biochemical origins, a yawn isn’t necessarily cause for offense.