The scientists have done a study. Money quote:
These results clearly suggest that yawning regulates the temperature inside your head, and that perhaps we yawn to cool our brains. What we don't have here is any biochemical cause-and-effect, but this is an incredibly interesting theory all the same.
The findings, if valid, draw a few unexpected conclusions about yawning. First of all, rather than stimulating sleep, a good yawn should fend off falling asleep. And secondly, far from being an indicator of boredom, yawning would appear to be a mechanism for maintaining attention.
So be sure and remember this for your next staff meeting -- it's an excellent excuse, and possibly even a valid one!