White noise

白噪音

White noise is a type of noise produced by combining sounds with different frequencies together.

科普时间：白噪音是由不同频率的声音组合在一起，而产生的“噪音”。

Buses are often crowded every day with people going to and from work. Therefore, they are the perfect environment for white noise. They produce consistent noises, like the hum of tires (轮胎) or the purring (嗡嗡声) of the engine. 

公交车常常挤满了人——轮胎发出的声音，还有引擎声……这里为白噪声提供了完美的环境。

White noise masks distracting sounds and is helpful for people who have trouble getting sleep.

这些声音掩盖了一些让人分心的声音——这对入睡有一定帮助。

Rocking motion

在车上摇摇晃晃

Another popular view on why it’s easy to fall asleep on buses is because of the rocking motion. “The motion of buses resembles (像) a mother swaying (摇晃) a baby in her arms. It is very comfortable,” Frankie Chow, founder of Ulu Travel Agency, told The Washington Post. 
关于为什么在公交车上容易睡着的一个观点——因为坐在车上会晃来晃去。“这种摇摆和母亲在臂弯里哄睡婴儿有点类似，让人感到舒适。”——Ulu旅行社创始人Frankie Chow在接受《华盛顿邮报》采访时表示。

However, it’s not exactly the rocking movement that induces sleep. Rather, it’s the association between rocking and sleeping. If you were rocked to sleep as a baby, you’d associate that rocking motion with going to sleep.

然而，并不完全是因为晃动会让你犯困——如果你在婴儿时期被摇晃着哄睡，你更可能会把这两者联系在一起。