Experiments have also cast serious doubt on the belief that yawning is triggered by a drop in blood oxygen or a rise in blood carbon dioxide.

对于打哈欠是由血液中的氧气减少或血液中的二氧化碳增加引起的这种观点，一些实验也同样提出了严重的质疑。

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However, there is now accumulating evidence that casts serious doubt on the existence of the phenomenon.

然而，越来越多的证据对此现象的存在提出了深深的怀疑。

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However, there is now accumulating evidence that casts serious doubt on the existence of the phenomenon.

然而，越来越多的证据对此现象的存在提出了深深的怀疑。

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