Most often, people yawn when they’re bored or tired, and we used to think yawning was a sign that the body needed more oxygen; however this is not the case anymore. A Physiology & Behavior journal found the involuntary reflex of yawning has more to do with regulating temperature in the brain.
The University of Vienna found that yawning depends on the temperature surrounding you, and a sharp intake of air can instantly cool down your brain. Therefore, yawning consistently occurs more often in hotter temperatures, and also when you're tired, because your brain struggles to maintain a constant temperature.
Meanwhile, the contagious yawn — we've all "caught" a yawn from someone, right? — is caused by sharing the other person's feelings. Researches at Baylor University found that more empathetic people contagiously yawn more often, and you're more likely to "catch" a yawn from a friend or family member than a stranger.