Asked by: Alex Dakin, Switzerland
No-one is sure why we yawn. We do know that yawning increases with levels of some of the brain’s neurotransmitters, including dopamine and serotonin, and decreases with levels of the opium-like endorphins.
One theory as to what triggers a yawn is that the long inhalation and short exhalation bring in more oxygen and reduce carbon dioxide, which might explain why we yawn when we are tired, bored or stuck in a stuffy room. But in fact, yawning is not an efficient way to increase oxygen levels, nor does giving people extra oxygen stop them yawning.
Other theories invoke temperature control, either of the whole body or just of the brain, which is especially sensitive and needs a steady temperature to function.
Yet another theory is that the stretching that often accompanies yawning (doing both at once is called ‘pandiculation’) keeps us ready for action. Infectious yawning is also thought to keep whole groups of animals alert together and synchronise their sleeping and waking.