32. Yawning Indicates Fatigue
Behavior scientists are particularly interested in yawning as a social contagion, but that is far from the only reason why we yawn. You are far more likely to yawn when you are tired, whether you are by yourself or surrounded by other people.
When you yawn, your body energizes itself by consuming copious amounts of oxygen and releasing the toxic by-product of respiration, carbon dioxide. Beyond that, scientists aren’t entirely sure why fatigue leads to excessive yawning. Maybe you are just less likely to be able to resist the impulse when it strikes, but there are likely many explanations.