Even though we yawn almost every day, scientists don't really know why we do it.
One theory suggests that people yawn when they need more oxygen in their blood. Another says that the body may yawn to wake itself up because yawning stretches the lungs and increases blood flow to the brain. Yet another theory says that yawning was used as a way for early humans to communicate, perhaps to tell each other they're bored or tired.
However, many researchers now believe that yawning may cool the brain when it gets too hot. Opening the mouth wide allows cool air to come into the body and moves blood around, which may help to carry heat away from the brain.
A 2007 study asked participants to watch a video of other people yawning while holding a towel to their forehead that was either 46 degrees Celsius, 4 degrees Celsius or room temperature. Researchers recorded nine yawns each from the groups holding the warm and room temperature towel, but only one from the group with the cold towel.
Another study from 2014 looked at whether people in Austria yawned more in summer or winter. Participants were asked to look through pictures of people yawning before taking a survey to say how often they yawned while doing so. The researchers found that 18% of participants yawned in winter, while in summer, almost 42% yawned.
Seeing someone else yawn, or even just reading about it, can make you yawn, too. Researchers don't know why yawns are so contagious, but it may be because if one person in a group needs to cool their brain, others may need to as well.
