Why do we Yawn?

Here's a puzzle that science has yet to solve; it's something we all do, it's contagious and even animals are affected. It is, of course, yawning. But why do we do it? We spoke to Gordon Gallup from the State University of New York at Albany...

Chris - What have you done to try to work out what a yawn is?

Gordon - We think that yawning may have evolved to function as a brain cooling mechanism. It turns out that a variety of drugs that inadvertently increase brain temperature, such as antidepressants, often produce excessive yawning.

Chris - When you say increase brain temperature, literally they make the brain hotter, but how?

Gordon - that's correct, they raise brain temperature, some are the serotonin re-uptake inhibitors that are used as antidepressants.

Chris - Is that because they make the brain more active, it burns more energy so it produces more heat?

Gordon - I suspect that it increases brain metabolism.

Chris - And a side effect of that increase in metabolism and temperature seems to be more yawning amongst sufferers.

Gordon - It seems to be the case. Likewise, sleep deprivation increases brain temperature and excessive yawning is a common symptom of sleep deprivation.

Chris - So how did you try and tease out whether it's just the temperature or something else going on?

Gordon - We focused on two well-established brain cooling mechanisms; it involves what's called "nasal breathing" and "forehead cooling". When you breathe in cool air through your nose, it cools the blood in capillaries in your nasal epithelium and sends that cooler blood to the brain. Likewise, when you cool your forehead there are emissary veins that are connected to your brain, and that cools your brain. So what we did is we had subjects either breathe through their noses or hold cold packs to their foreheads and we discovered that under those conditions yawning stopped.

Chris - How did you make them yawn?

Gordon - we made them yawn by having them watch videotapes of other people who were yawning.

Chris - So this was contagious yawning?

Gordon - Contagious yawning, exactly. By cooling the brain presumably it eliminated the need to yawn and as a consequence you don't get contagious yawning under those conditions.

Chris - So are there any other explanations apart from the temperature-related idea?

Gordon - Well, a lot of people think that you yawn in order to replenish oxygen levels in the blood, but a lot of independent research has been done on that question and it turns out that if you manipulate both oxygen and CO2 levels in a persons blood, it leaves yawning unaffected.

Chris - Now when you say you got people to cool their brain via their forehead, tell us a bit about that, how did that work?

Gordon - Well, we had them either hold warm packs to their forehead, cold packs to their forehead or packs that were maintained at room temperature; and those that held cold packs to their forehead stopped yawning.

Chris - You don't think that's because it's actually quite unpleasant having something cold jammed onto your forehead, and this made the subjects more aroused and more alert, just because they were doing something which could be quite painful?

Gordon - Well, they weren't so cold as to produce painful after-effects, but the fact that nasal breathing produced the same effect tends to rule out any discomfort.

Chris - Well lets look at the group dynamic then; why is it that if we're sitting in a cluster together, I yawn, the person next to me might catch it and yawn, and probably the whole audience to this programme are now yawning in sympathy with us talking about it, and not just because they're bored. Why should that happen? What's the evolutionary purpose? There must be one for animals to do it too.

Gordon - We think that contagious yawning evolved to maintain group vigilance, so as to enable people to be better at detecting danger. That is, when someone yawns, we take that as evidence for the fact that their brain temperature is increased and their mental efficiency is decreased. Therefore, if everybody yawns the overall level of vigilance on the part of the group is enhanced.

Chris - And so in that respect, you're more likely to spot that tiger lurking which is going to pluck you off when you're all asleep

Gordon - Precisely.