Yawning involves opening the mouth involuntarily while taking a long, deep breath of air. It is commonly believed that people yawn as a result of drowsiness or weariness because they need oxygen.

However, researchers at the University of Albany in New York said their experiments on 44 students showed that drawing in air helps cool the brain and helps it work more effectively.

They said that their experiments showed that raising or lowering oxygen and carbon dioxide levels in the blood did not produce that reaction.

Study participants were shown videos of people laughing, being neutral and yawning, and researchers counted how many times the volunteers responded to their own 'contagious yawns,' reported the online edition of BBC News.

The researchers found that those who breathed through the nose rather than the mouth were less likely to yawn when watching a video of other people yawning. This was because vessels in the nasal cavity sent cool blood to the brain.

The same effect was found among those who held a cool pack to their forehead, whereas those who held a warm or room-temperature pack yawned while watching the video.

'Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain optimal levels of mental efficiency,' the authors wrote in the journal Evolutionary Psychology.

最新研究表明，打呵欠有助于保持头脑清醒，这与“打呵欠促使人进入睡眠状态以及是疲倦的一种表现”的普遍看法正好相反。

打呵欠使人在不由自主张开嘴的同时，能长长的、深深的吸一口气。普遍看法认为，人们打呵欠是由于困乏或疲倦造成的，因为他们需要氧气。

然而，据纽约奥尔巴尼大学的研究人员介绍，他们对44名学生的实验表明，吸入空气有助于醒脑，从而使它更为有效的工作。

研究人员称，实验表明，提高或降低血液中氧气和二氧化碳的含量不会导致那种反应。

据BBC新闻网报道，研究人员先让实验对象观看人们大笑、正常状态以及打呵欠的视频，然后计算出志愿者“传染性打呵欠”的次数。

研究人员发现，在看别人打呵欠的视频时，用鼻子呼吸的人比用口腔呼吸的人打呵欠的几率要小。这是因为鼻腔里的血管将温度较低的血液送到了脑部。

将冰袋敷在前额上也能起到类似的效果，而在观看视频时用暖水袋或室温水袋敷在额头上的人却无法避免打呵欠。

这份在《进化生理学》上发表的研究报告提到，“由于脑部温度上升时就会打呵欠，所以将低温血液送至大脑能够保持脑部效率的最佳水平。”