It was long believed you yawned when there was too much carbon dioxide and not enough oxygen in your blood.A part of your brain called the brain stem detected this and triggered the yawn reflex.

• 很长一段时间，人们都相信打哈欠是因为血液里二氧化碳太多而氧气不足，大脑茎干觉察到这点于是触动了打哈欠的神经。