Yawning is a semi-voluntary action performed by pretty much all vertebrates. It consists of three phases: a long inspiratory phase, a brief acme, and finally rapid expiration. This is accompanied by many physiological and neurological changes, such as muscular stretching, increase of blood flow and a sense of pleasure with dopaminergic activity in certain regions of the brain (Daquin et al., 2001). Yawning does not, as such, affect arousal in a physiological sense (Guggisberg et al., 2007), but it is obviously associated with boredom, waking up and feeling sleepy. As everyone knows very well, yawning is also contagious. This is not true just for humans, this is true for all social animals who have a sense of self-awareness (i.e. can recognize themselves in the mirror) (Perriol et al., 2006).
Why do we yawn? There are so many different changes associated with yawning that it could really be anything. Hippocrates considered it to be a mechanism for exhaustion of the fumes preceding fever. For the largest part of the 20th century, it was commonly thought that it was a mechanism for "balancing" oxygen and carbon dioxide levels, until this was shown to be untrue in 1987 (Provine et al.). There is obviously some connection with the states of sleep and wakefulness, but is that all? Also, why is it contagious? Is it just a mirror neuron driven response, or does it have a useful function?
The evidence for the connection between yawning and the sleep-wake rhythm is quite convincing. Even though this is generally common knowledge, Zilli et al. (2007) showed that yawning occurs more frequently in the early morning and late evening. Also, it was shown that evening-types (people who tend to stay up late, like myself) yawn more frequently than morning-types, showing that there is a connection between changes in the sleep-wake rhythm and yawning.
Another interesting connection is that between yawning frequency and REM sleep as observed throughout life. Humans can yawn from as early as 12 weeks after conception. Yawning frequency then very slowly declines with age. There is a very similar pattern in the amount of REM sleep, along with some physiological connections, such as the muscle stretching in yawning counteracting the muscular atonia seen during REM sleep (Walusinski, 2006).
The contagiousness of yawning is, for most people, its most interesting aspect. Anderson et al. (2004) showed that yawning is contagious in chimps, while Paukner et al. (2006) showed that the same is true in stumptail macaques. Infant chimps exhibited no yawning in response to the same experiments, which is in line with the evidence that human children under the age of 5 do not respond to seeing a person yawn like older humans do. Considering this and the fact that only animals with a sense of self-awareness exhibit contagious yawning, could contagious yawning have some connection with intelligence or social interaction?
Another aspect of contagious yawning is that it is correlated with interoception (sensitivity to stimuli arising within the body), self-awareness and empathy, as well as mental state attribution (the ability to inferentially model the mental states of others) (Platek et al., 2003). Functional MRI shows that brain activity correlated with contagious yawning is significant in the bilateral precuneus and posterior cingulate, regions highly associated with self-processing but, surprisingly, there is no activity in regions associated with mirror neurons (Platek et al., 2004). Yawning seems to circumvent the mirror neuron system, suggesting that yawning is an automatically released behavioural act and not an imitated motor pattern requiring detailed understanding (Schurmann et al., 2004).
The most surprising data comes from a 2007 study by Gallup et al. which suggests that yawning is a mechanism for thermoregulation of the brain. The response of human subjects to videos of people yawning was observed. Subjects breathing through the mouth exhibited 48% contagious yawning, while subjects instructed to breathe nasally did not yawn at all. Nasal breathing is associated with brain thermoregulation by the cooling of the vertebral artery and the forebrain. Also, subjects with a cool compress applied to their foreheads yawned significantly less than subjects with no compress or a warm compress applied. This leads to the formulation of an interesting theory with testable predictions about the thermoregulatory function of yawning based on the rapid intake of cool air. The authors predict that as ambient temperature approaches body temperature, yawning should diminish, and when ambient temperature exceeds that of the body, yawning should stop. This might also explain why subjects with a warm compress applied to their forehead did not yawn more, since the body might detect the high temperature compress and interpret it as higher ambient temperature.
We are getting quite close to a Unified Theory of Yawning. Even though the physiology and neurology behind it is very complex, we seem to be moving in the right direction. We can even make some evolutionary hypotheses about yawning without sounding entirely speculative. For example, we might say that a population in which yawning is contagious will be more synchronised and better able to maintain its function as a unit, whether that involves moving around at specific times of day, making sure no-one wanders off after bedtime or keeping safe from enemies. Hopefully all the yawning you just did while reading this article was only because just thinking about it makes you yawn.
Andrew C. Gallup, Gordon G. Gallup Jr. (2007). Yawning as a Brain Cooling Mechanism: Nasal Breathing and Forehead Cooling Diminish the Incidence of Contagious Yawning Evolutionary Psychology, 5 (1), 92-101
Also, a website entirely dedicated to yawning called Baillement, which is French for "yawn". Lots of papers to read with some comments by the website authors themselves.