你爲什麼要睡覺?睡覺是爲了遺忘!

Sleep is as mysterious as it is vital for our wellbeing. Over the decades, researchers have proposed several mechanisms through which sleep rejuvenates us, but we still don't fully understand the big picture. Now, two recently published studies come up with an interesting explanation: we sleep to forget some of the things we learn during the day.

睡眠對我們的健康至關重要，但它神祕莫測。過去的數十年裏，研究人員提出了若干種睡眠讓人煥然一新的機理，但我們依然沒有完全掌握有關睡眠的全部知識。現在，兩篇新近發表的研究提出了一個有趣的解釋：我們入睡是爲了忘掉白天學到的某些東西。

We store memories in networks in our brains. Whenever we learn something new, we grow new connections between neurons, called synapses. In 2003, Giulio Tononi and Chiara Cirelli, biologists at the University of Wisconsin-Madison, proposed something very interesting: during the day, we learn so much and develop so many synapses that things sometimes get fuzzy. Since then, the two and their collaborators have made quite a few interesting additions to that study.

我們通過大腦網絡存儲記憶。無論何時學到新東西，神經元之間都會建立起一種叫做“突觸”的聯結。2003年，威斯康星大學麥迪遜分校的生物學家朱里奧•託諾尼和基婭拉•西勒利提出了一種很有意思的觀點：我們在白天學到了那麼多東西，構建了那麼多突觸，有時候把自己搞糊塗了。從那時起，這兩位生物學家就和同事們一起在這項研究的基礎上不斷添加了許多有趣的內容。

For starters, they showed that neurons can prune out some synapses, at least in the lab. But they suspected the same things happens every day, naturally, in our brains — probably during sleep.

首先，他們告訴大家：神經元會修剪掉一些突觸，至少在實驗室裏是這樣的。但他們猜測同樣的事也會每天在自然狀態下的大腦中發生，而且很可能是在睡眠中發生。

So they set up a painstaking experiment, in which Luisa de Vivo, an assistant scientist working in their lab, collected 6,920 synapses from mice, both awake and sleeping. Then, they determined the shape and size of all these synapses, learning that the synapses in sleeping mice were 18 percent smaller than in awake ones. That's quite a big margin.

所以，他們設置了一項非常費力的實驗：他們實驗室中的一位助理研究員路易莎•德•維沃從老鼠身上找了6920個突觸，這些老鼠有的醒着有的睡着。然後，他們弄清所有這些突觸的形狀和大小，發現睡着的老鼠身上的突觸要比醒着的那些小18%，差距非常大。

After this, they designed a memory test for mice. They placed the animals in a room where they would get a mild electrical shock if they walked over one particular section of the floor. They injected some of the mice with a substance that had been proved to prevent the pruning of new synapses. The mice that experienced this were much more likely to forget about the section and after a good night's sleep, they tended to walk over the section again, while mice that slept normally remembered better.

然後，他們爲老鼠設計了一項記憶測試。他們把老鼠放在一個房間裏，如果它們走到地板上一個特定區域，就會受到輕微電擊。他們往一些老鼠體內注射了一種物質，經驗證，這種物質能阻止神經元修剪新突觸。接受過注射的老鼠更容易忘記電擊區域，好好睡過一夜之後，它們還是會走到那個區域，而正常睡眠的老鼠則記得更清楚。

Then, Dr. Tononi and his colleagues found that the pruning didn't strike every neuron. Some 20% were unchanged, likely well-established memories that shouldn't be tampered with.

這樣，託諾尼博士和同事們就發現了：修剪並不會牽涉到每個神經元。大約有20%的部分不會改變，很可能牢固構建的記憶不會竄改。

In other words, we sleep to forget — but in a smart way.

換句話說，我們睡着是爲了遺忘——但是是以一種聰明的方式遺忘。