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.

　　睡眠对我们的健康至关重要，但它神秘莫测。过去的数十年里，研究人员提出了若干种睡眠让人焕然一新的机理，但我们依然没有完全掌握有关睡眠的全部知识。现在，两篇新近发表的研究提出了一个有趣的解释：我们入睡是为了忘掉白天学到的某些东西。

　　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.

　　我们通过大脑网络存储记忆。无论何时学到新东西，神经元之间都会建立起一种叫做“突触”的联结。2003年，威斯康星大学麦迪逊分校的生物学家朱里奥•托诺尼和基娅拉•西勒利提出了一种很有意思的观点：我们在白天学到了那么多东西，构建了那么多突触，有时候把自己搞糊涂了。从那时起，这两位生物学家就和同事们一起在这项研究的基础上不断添加了许多有趣的内容。

　　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，他们告诉大家：神经元会修剪掉一些突触，至少在实验室里是这样的。但他们猜测同样的事也会每天在自然状态下的大脑中发生，而且很可能是在睡眠中发生。

　　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，他们设置了一项非常费力的实验：他们实验室中的一位助理研究员路易莎•德•维沃从老鼠身上找了6920个突触，这些老鼠有的醒着有的睡着。然后，他们弄清所有这些突触的形状和大小，发现睡着的老鼠身上的突触要比醒着的那些小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.

　　然后，他们为老鼠设计了一项记忆测试。他们把老鼠放在一个房间里，如果它们走到地板上一个特定区域，就会受到轻微电击。他们往一些老鼠体内注射了一种物质，经验证，这种物质能阻止神经元修剪新突触。接受过注射的老鼠更容易忘记电击区域，好好睡过一夜之后，它们还是会走到那个区域，而正常睡眠的老鼠则记得更清楚。

　　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.

　　这样，托诺尼博士和同事们就发现了：修剪并不会牵涉到每个神经元。大约有20%的部分不会改变，很可能牢固构建的记忆不会窜改。

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

　　换句话说，我们睡着是为了遗忘——但是是以一种聪明的方式遗忘。

（编辑：何莹莹）