Unborn babies yawn in the womb. Children and adults don't just yawn when they are tired or bored, but also before becoming active. We yawn when we go to sleep and when we wake up. The actual cause of yawning remains a mystery, and experiments with oxygen masks have shown that it has nothing to do with a lack of oxygen. It seems likely that yawning is a social signal that invites others to join in with whatever we want to do, whether it's some communal activity or simply going to sleep. Around half of those who see another person yawning will do the same, so yawning really is infectious. People who find it very easy to empathize with others are among the most affected.