Though you may not pay attention to it unless you're sick, you're always breathing more heavily from one nostril¹ than the other.

除非生病了，否则你可能并不会注意到，你的一个鼻孔总是比另外一个呼吸更费力。

During the day, the sides switch and the other nostril goes into 'work mode', but why?

一天之中，两边的鼻孔可能会交换，另一侧的鼻孔进入“工作模式”。但这是为什么呢？

This process is automated² by the aptly named autonomic nervous system, which is the same system that controls many things your body does all by itself such as digestion³ and heart rate.

这一过程由自主神经系统自动控制，这个系统也控制着你身体自动运转的许多方面，比如消化和心率。

For your nose, this system controls your 'nasal cycle', so that each nostril operates effectively.

对你的鼻子而言，这一系统控制着你的“鼻子循环”，便于每个鼻孔有效工作。

The nasal cycle happens, according to the US National Library of Medicine, several times during the day, and is only brought to your attention if your nose is clogged⁴ up more than usual.

据《美国国家医学图书馆》透露，鼻子循环一天会出现好几次。只有当你的鼻子比平时更堵的时候，你才会注意到这一点。

In order to open one side of your nose and close the other, your body inflates⁵ tissue with blood in your nose.

为了打开一边的鼻孔、关闭另一边，你的身体会让鼻子里的组织充血膨胀。

"Increased blood flow causes congestion⁶ in one nostril for about 3 to 6 hours before switching to the other side. There is also increased congestion when one is lying down, which can be especially noticeable when the head is turned to one side," Jennifer Shu reports for CNN.

据美国有线新闻网的詹妮弗-舒报道：“血流增加会导致一边的鼻孔堵塞3-6个小时，之后才会转移到另一边。人们躺下来的时候，鼻孔堵塞程度也会增加。人们将头转向一边的时候，这一点会格外明显。”

It's believed that this cycle helps round out your sense of smell. As Matt Soniak reports for MentalFloss, some smells are better picked up by fast moving air through your nose, while others take more time and are detected better with slow-moving air.

据称，这种循环有助于丰富人们的嗅觉。据MentalFloss网站的马特-索尼亚克报道说，某些气味能更好地被鼻孔中快速气流捕捉到，而其它气味需要更多时间被慢速气流捕捉到。

If one side of your nose is wide open and the other is slightly closed, you get all of the smells.

如果一个鼻孔张得很大，而另外一个鼻孔略微闭合，那么你就能够闻到所有的气味了。

The process also gives each side of your nose a break, since a constant stream of heavily flowing air can dry it out and kill off the small hairs that protect you from foreign contaminants.

这一过程也能让你的两个鼻孔稍微休息一下，因为如果不断有大量空气流过鼻孔，会导致鼻孔干燥并损害鼻毛，鼻毛能够保护你不受外来污染物伤害。

When you get sick, the whole process can become unbearable⁷, because the one nostril that is effectively 'turned off' feels way, way more clogged than the other, Soniak says.

索尼亚克说，当你生病的时候，这一过程会变得令人难以忍受，毕竟其中一个有效“关闭”的鼻孔会让你感觉比另一侧更堵。

Basically, the clogged-up feeling is just amplified⁸ by the cold.

基本上，感冒会使鼻子堵塞的感觉放大。

So the next time you feel like you're only breathing from one side of your nose, remember that it's a natural, automatic system working to keep you smelling properly, and to make sure your nose doesn't get dried out by a constant onslaught of dirty air.

下次，你感觉自己只能用一侧鼻孔呼吸的时候，请记住这是一件很正常的事。你的自主系统正在帮助你恰当地闻到气味，并确保你的鼻子不会因为不断呼吸肮脏的空气而变得干燥。