Yawning when tired is a complex reflex likely serving multiple biological functions. One leading theory is that yawning helps cool the brain. As the brain’s temperature rises with fatigue, yawning induces a heat exchange by opening the mouth and deeply inhaling, thus increasing blood flow and bringing cooler air to the brain, refreshing it like a natural air conditioner. This cooling helps maintain optimal brain efficiency during tiredness. Another perspective involves neurotransmitters in the brain, such as dopamine and serotonin, which fluctuate during fatigue and may trigger yawns to help the brain recalibrate and regain alertness. While older ideas suggested yawning increased oxygen to the blood, research shows oxygen levels are likely not the main cause. Additionally, yawning involves stretching jaw and face muscles and activating certain brain areas that play roles in alertness and emotion, indicating yawning is a complex mechanism for signaling changes in alertness or readiness in the body. It is both a physical and neurological response to tiredness that promotes brain function and communication of the body's state.