*Yawn*

Rather than being a precursor to sleep, yawning is designed to keep us awake, say US researchers. But why does seeing someone else yawn make you to do the same?

Yawning is an involuntary action that everyone does. We start before we are born and most creatures on the planet do it - even snakes and fish. New research suggests rather than being a precursor to sleep, the purpose of yawning is to cool the brain so it operates more efficiently and keeps you awake. The theory could explain a puzzling question about subconscious human behaviour - why many of us yawn when we see or hear another person doing it, or even read about it or even just think about it?

The brain cooling theory says that when we contagiously yawn we are participating in an ancient, hardwired ritual that evolved to help groups stay alert and detect danger.

Why is yawning contagious? [BBC]