We yawn when we are tired primarily because yawning helps regulate brain temperature and maintain alertness. One leading theory suggests that yawning cools the brain, much like a fan cools a laptop, by opening the jaw and stretching muscles to increase blood flow and bring cooler air into the brain. This cooling helps prevent the brain from overheating, which can happen when we feel drowsy or fatigued. Yawning may also serve to wake the body up by stretching the lungs and muscles and increasing blood flow to the brain and face, helping improve alertness when tired. Additionally, yawning occurs during times when our state of alertness is changing, such as waking up or feeling fatigued, indicating it may signal changes in bodily conditions. Some research suggests yawning might also have social or communicative functions related to empathy, as yawns are contagious and may synchronize group behavior, but the main physiological role when tired appears to be brain cooling and maintaining alertness. In short, yawning when tired helps cool the brain and stimulate wakefulness to counteract fatigue.
