在身体上，打哈欠包括张开嘴，吸入空气，打开下巴，拉伸耳膜和呼气。它可能由疲劳，无聊，压力或看到别人打哈欠引发。因为它是一种反射，打哈欠涉及与疲倦，食欲，紧张和情绪相关的神经递质的相互作用。这些化学物质包括一氧化氮，血清素，多巴胺和谷氨酸。科学家们知道某些医疗条件（例如，多发性硬化症，中风和糖尿病）会改变打哈欠频率和打哈欠后唾液中皮质醇的水平。因为打哈欠是神经化学的问题，所以有几种可能的原因。在动物中，其中一些原因很容易理解。例如，蛇在吃完后打哈欠以重新调整下颚并帮助呼吸。当他们的水缺乏足够的氧气时，鱼打哈欠。确定为什么人类打哈欠更难确定。因为打哈欠后皮质醇水平增加，它可能会提高警觉性并表明需要采取行动。心理学家Andrew Gallup和Gordon Gallup相信打呵欠有助于改善大脑的血流量。前提是拉伸颌部增加血液流向面部，头部和颈部，而打哈欠的深呼吸迫使血液和脊髓液向下流动。这种打哈欠的物理基础可以解释为什么人们在焦虑或压力时打哈欠。伞兵在退出飞机前打哈欠。
On the body, yawning involves opening your mouth, inhaling air, opening your chin, stretching the eardrum and exhaling. It can be caused by fatigue, boredom, stress or seeing others yawn. Because it is a reflection, yawning involves the interaction of neurotransmitters associated with fatigue, appetite, tension and mood. These chemicals include nitric oxide, serotonin, dopamine and glutamic acid. Scientists know that certain medical conditions (eg, multiple sclerosis, stroke, and diabetes) alter the frequency of yawning and the level of cortisol in saliva after yawning. Because yawning is a neurochemical problem, there are several possible reasons. Some of the reasons in animals are easy to understand. For example, the snake yawns after eating to re-adjust the squat and help with breathing. When their water lacks enough oxygen, the fish yawns. Determining why humans yawn is harder to determine. Because cortisol levels increase after yawning, it may increase alertness and indicate the need for action. Psychologists Andrew Gallup and Gordon Gallup believe that yawning helps improve blood flow to the brain. The premise is that stretching the jaws increases blood flow to the face, head and neck, while deep breathing of yawns forces the blood and spinal fluid to flow downward. The physical basis of this yawn can explain why people yawn when they are anxious or stressed. The paratrooper yawned before exiting the plane.