When I say this word, you will begin to feel very sleepy. In fact, yawning is so contagious that even the sight of the word "yawn" is enough to make six out 10 of us gape and bear our molars.

We quickly reach up to cover our mouths, hoping we won't offend our colleagues. But should we? Is it really rude to yawn?

It may be associated with sleepiness and boredom, but paratroopers report yawning before they jump, and Olympic athletes have been seen yawning before they begin a race. Are they feeling fatigued? It seems unlikely.

Over the years, there have been many theories for why people yawn. Some say it clears the tonsils after eating, others that it equalises ear pressures. The most commonly given explanation - that it increases the amount of oxygen in the blood - has been widely discredited.

The truth is that, even in the age in which the human genome has been sequenced and space travel has become almost passé, we still have no idea why we yawn, nor why it is so contagious.

Common sense tells us we yawn when we are tired, so it may have something to with keeping us alert. A study by US experts has found that when you yawn, the inhaled air reduces the temperature of vessels in the nasal cavity, allowing cooled blood to be sent to the brain.

This chills the brain, making it more alert and effective. So if you're looking to avoid embarrassment and quell a fit of the yawns, simply take a deep nasal inhale and let the brainfreeze revive you.

But why do we "catch" others' yawns even when we are not tired ourselves? One person's drowsy, gaping mouth can set off a room of other people. Research in Finland suggests that 60% of us yawn when we see someone else doing it, even when they are not tired. Yawn and the whole world yawns with you.

According to new research, this contagious yawning is all about empathy, and how human beings have the ability to put themselves in someone else's shoes. Previous tests have shown that people who perform well in assessing other people's moods are more likely to "catch" a yawn; now, in a new study, researchers at University College London have found evidence to back this up.

Atsushi Senju showed videos of people yawning or making other mouth movements to 24 children with an autistic-spectrum disorder, and to 25 normally developing children. Both groups of children yawned about the same amount while watching the general mouth movements. But when shown a video of yawning, the normally developing children yawned much more often. The autistic group did not increase their yawning frequency, according to the report in the journal Biology Letters.

In short, yawners are not rude: they are deeply considerate, understanding types. Next time your partner yawns at you, it's not because your chat is boring. Quite the opposite - it's a sign that they have a deep understanding of the hard work you've been doing to keep them happy.

That's all very sweet, but what benefit is there to this synchronisation? Well, some scientists say it could be essential for the survival of the species. They believe that contagious yawning is similar to animals' herd instincts - like when a flock of birds rises in tandem with one individual that has noticed a predator. In the early days of human evolution, yawning might have helped groups bond together, according to the psychologist Dr James Anderson of Stirling University. For example, he says, if a group are sitting around the campfire and the leader yawns, it serves as a signal to others in the group that it is time to retire. Those who are supportive of the leader may be more inclined to yawn. Alternatively, it may simply act as a warning for the rest of the group to remain alert.

One attractive theory is that yawning simply gives us pleasure. Indeed, it rates 8.5 on the 10-point "hedonic scale". And yawning is actually sexy, according to Dutch academic Wolter Seuntjens. In his thesis, The Hidden Sexuality of the Human Yawn, he describes the feeling that accompanies a yawn as a "mini-orgasm". And when you consider that the average yawn lasts about six seconds, that can't be bad, can it?