PARIS (Reuters Life!) - We do it when we're tired, when we're bored or when we're hungry; parachutists have been seen to do it before a jump, and research has even suggested a link between yawning and sexual arousal.
But the exact causes and function of yawning remain a mystery, and one that until recently was surprisingly under-documented in the scientific world.
Now a French family doctor, Olivier Walusinski, has published what is billed as the first ever textbook on the subject, "The Mystery of Yawning in Physiology and Disease" -- a collection of the latest research on this baffling and uncontrollable behavior.
The book will be followed up on June 24-25 with the First International Conference on Yawning in Paris, which will address issues such as the role of yawning as a brain-cooling mechanism and the hidden sexuality of the yawn.
"There are a number of theories, but there's no formal proof as yet of why we yawn," Walusinski told Reuters.
What is known is that the average human will yawn around 250,000 times over the course of his life, and that babies in the womb do it from as early as 12-14 weeks, suggesting it plays an important neurophysiological role.
"If a fetus weighing just 60 g (2.116 oz) can expend the amount of energy needed to yawn and stretch, then it must be absolutely vital to its development," Walusinski said.
Birds do it, fish do it, in fact nearly all cold and warm-blooded vertebrates do it, with the exception of giraffes and whales which have not yet been caught gaping involuntarily.
In humans, yawning is still widely believed to increase oxygen levels in the blood and eliminate excess carbon dioxide, but this theory was ruled out as far back as the 1980s.
"The idea dates back to the 17th century, but studies by the American Robert Provine... showed that the concentrations of gases in the blood remained exactly the same before and after yawning," said Walusinski.
Instead, the fact we yawn when we're sleepy or bored has led recent research to suggest it is used to increase vigilance.
Yawning when hungry is thought to support this theory although, unlike lions and other carnivores, humans no longer need heightened instincts to hunt down their prey.
As for why parachutists yawn before a death-defying jump, this again could be to increase vigilance, but there is also speculation it could help counter a rise in stress.
Rats subjected to stressful stimuli in cages have certainly been found to yawn more frequently, however exactly what function their mouth gaping serves has yet to be shown.
Another theory is that yawning helps to cool the brain, protecting cerebral activity and boosting efficiency.
But before you use this excuse in the next boring meeting, it's worth noting that the idea raises a number of questions, such as why we don't yawn more during a fever, and why snakes yawn when they have no temperature regulating mechanism.
What is certain is that yawning is catching, Walusinski said, and this has been linked to empathy in humans.
Neuroimagery has shown the same parts of the brain light up when we feel empathy as when we "catch" a yawn, while personality tests have shown people with schizophrenia or autism are less likely to be affected.
A Dutch researcher has also suggested a link between yawning and sexuality in humans, based on circumstantial evidence such as representations of yawning in literature and the visual arts.
Popular imagery, for example, frequently shows a gaping mouth, stretching and thrusting out of the chest as an erotic posture in women, supporting the idea that yawning has a sexual side and could indeed be an invitation to sex.
In animals, Walusinski noted, the link is far more obvious.
"In macaque monkeys, the dominant male yawns before and after mating, and this is testosterone-dependent," said Walusinski.
Castration, he said, causes the male to lose his dominant status, and the yawning stops.
Editing by Paul Casciato