当我们感到疲倦时,体内已产生许多的二氧化碳。当二氧化碳过多时,必须再增加氧气来平衡体内所需。因为这些残留的二氧化碳会影响我们身体的机能活动,这时身体便会发出保护性的反应,于是就打起哈欠来。打哈欠是一种深呼吸动作,它会让我们比平常更多地吸进氧 When we feel tired, the body has produced a lot of carbon dioxide. When carbon dioxide is too high, oxygen must be added to balance the body’s needs. Because these residual carbon dioxide can affect the performance of our physical activity, then the body will issue a protective reaction, so yawn. Yawning is a deep breathing action that will allow us to inhale more oxygen than usual