人们往往认为打呵欠是因为困倦或疲惫。美国研究人员发现,打呵欠可能也是大脑过热的迹象,人通过打呵欠吸入外界空气给大脑降温。
美国普林斯顿大学生态学和进化生物学学院博士后安德鲁·盖洛普带领研究小组,研究季节对人打呵欠的影响。研究人员在亚利桑那州图森市随机拦住80名行人,向他们展示打呵欠的图片,观察行人是否跟着打呵欠。测试分别在冬季和夏季各进行一次。结果显示,冬季一半研究对象跟着打呵欠,夏季只有四分之一研究对象“受传染”。
美国每日科学网站9月20日援引盖洛普的话报道,过去数十年中,研究人员对打呵欠的控制机制提出诸多假设,但鲜有研究揭示打呵欠的生物学功能,至于打呵欠的目的或作用尚没有一致结论,“这项研究给打呵欠的控制机制涉及体温调节生理机能这种观点提供了更多支持”。
这一研究结果由本月出版的《进化神经学前沿》发表。
根据体温调节假设,打呵欠由大脑温度升高引起。先前研究结果显示,在打呵欠前后,小鼠大脑温度发生变化。打呵欠时,下颌拉伸增加了流向大脑的血流,这些血流与吸入的外界空气进行热量交换,最终帮助大脑降温。
按照这一假设,打哈欠应有反作用,即当外界空气温度等于或高于体温时,打呵欠吸入的空气无法冷却大脑。
这在盖洛普的研究中得到印证。夏季外界温度等于甚至高于体温,无助于让过热的大脑休息,因此研究对象打呵欠频率降低。
按照体温调节假设,当环境温度处于一定范围时,打呵欠频率最高。
盖洛普说,他们的研究结果与假设一致。综合考虑湿度、置身室外时间、睡眠时间等因素后,无论夏季还是冬季,研究对象初到室外5分钟内,近40%打呵欠,随后,夏季研究对象打呵欠比例降至10%以下,冬季打呵欠比例升高较多。
他说:“这是第一份显示打呵欠频率随季节变化的报告。”
研究人员认为,这项研究增加了人们的生理知识,有助解释人在高热环境中容易思维混乱、不知所措的原因,有助更好地理解运动神经元疾病或癫痫患者缘何常打呵欠。(生物谷 Bioon.com)
doi:10.3389/fnevo.2011.00003
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Contagious yawning and seasonal climate variation
Andrew C. Gallup and Omar T. Eldakar
Recent evidence suggests that yawning is a thermoregulatory behavior. To explore this possibility further, the frequency of contagious yawning in humans was measured while outdoors in a desert climate in the United States during two distinct temperature ranges and seasons (winter: 22oC; early summer: 37oC). As predicted, the proportion of pedestrians who yawned in response to seeing pictures of people yawning differed significantly between the two conditions (winter: 45%; summer: 24%). Across conditions yawning occurred at lower ambient temperatures, and the tendency to yawn during each season was associated with the length of time spent outside prior to being tested. Participants were more likely to yawn in the milder climate after spending long periods of time outside, while prolonged exposure to ambient temperatures at or above body temperature was associated with reduced yawning. This is the first report to show that the incidence of yawning in humans is associated with seasonal climate variation, further demonstrating that yawn-induced contagion effects can be mediated by factors unrelated to individual social characteristics or cognitive development.