Prolonged sleep deprivation can lead to severe health problems in humans and other animals. But why is sleep so vital to our health? A UCLA-led team of scientists has made a major advance in answering this question and has shown for the first time that a dramatic change in the purpose of sleep occurs at the age of about 2-and-a-half.

长期睡眠不足会导致人类和其他动物出现严重的健康问题。但是为什么睡眠对我们的健康如此重要呢?由加州大学洛杉矶分校领导的科学家团队在回答这个问题上取得了重大进展，并首次表明，在孩子大约2岁半的时候，睡眠的目的会发生戏剧性的变化。

Before that age, the brain grows very rapidly. During REM sleep, when vivid dreams occur, the young brain is busy building and strengthening synapses — the structures that connect neurons to one another and allow them to communicate.

在这个年龄之前，大脑发育非常迅速。在快速眼动（REM）睡眠期间，当生动的梦发生时，年幼的大脑正忙于建立和加强突触——连接神经元并使它们能够交流。

“Don’t wake babies up during REM sleep — important work is being done in their brains as they sleep,” said senior study author Gina Poe, a UCLA professor of integrative biology and physiology who has conducted sleep research for more than 30 years.

加州大学洛杉矶分校综合生物学和生理学教授Gina Poe从事睡眠研究已有30多年，她是这项研究的资深作者，她说:“在快速眼动睡眠期间，不要叫醒婴儿——在他们睡觉的时候，他们的大脑正在做重要的工作。”

After 2-and-a-half years, however, sleep’s primary purpose switches from brain building to brain maintenance and repair, a role it maintains for the rest of our lives, the scientists report Sept. 18 in the journal Science Advances. This transition, the researchers say, corresponds to changes in brain development.

然而，在两年半之后，睡眠的主要目的从大脑的构建转变为大脑的维护和修复，这是它在我们以后的生活中所扮演的角色，科学家在9月18日的《科学进展》（Science Advances）杂志上报道。研究人员说，这种转变与大脑发育的变化相对应。

All animals naturally experience a certain amount of neurological damage during waking hours, and the resulting debris, including damaged genes and proteins within neurons, can build up and cause brain disease. Sleep helps repair this damage and clear the debris — essentially decluttering the brain and taking out the trash that can lead to serious illness.

所有的动物在清醒的时候都会自然地经历一定程度的神经损伤，而由此产生的碎片，包括神经元内受损的基因和蛋白质，会累积并导致大脑疾病。睡眠有助于修复这种损伤，清理垃圾——本质上就是清理大脑，清理掉那些可能导致严重疾病的垃圾。

Nearly all of this brain repair occurs during sleep, according to senior author Van Savage, a UCLA professor of ecology and evolutionary biology and of computational medicine, and his colleagues.

加州大学洛杉矶分校的生态、进化生物学和计算医学教授、资深作者Van Savage和他的同事们认为，几乎所有的大脑修复都是在睡眠中进行的。

“I was shocked how huge a change this is over a short period of time, and that this switch occurs when we’re so young,” Savage said. “It’s a transition that is analogous to when water freezes to ice.”

Savage说:“在这么短的时间内就发生了这么大的变化，而且这种变化发生在我们这么年幼的时候，这让我很震惊。这种转变类似于水结冰的过程。”

The research team, which included scientists with expertise in neuroscience, biology, statistics and physics, conducted the most comprehensive statistical analysis of sleep to date, using data from more than 60 sleep studies involving humans and other mammals. They examined data on sleep throughout development — including total sleep time, REM sleep time, brain size and body size — and built and tested a mathematical model to explain how sleep changes with brain and body size.

该研究团队包括在神经科学、生物学、统计学和物理学方面有专长的科学家，利用60多项涉及人类和其他哺乳动物的睡眠研究数据，对睡眠进行了迄今为止最全面的统计分析。他们检查了发育过程中的睡眠数据——包括总睡眠时间、快速眼动睡眠时间、大脑大小和身体大小——并建立和测试了一个数学模型来解释睡眠是如何随着大脑和身体大小变化的。

The data were remarkably consistent: All species experienced a dramatic decline in REM sleep when they reached the human developmental equivalent of about 2-and-half years of age. The fraction of time spent in REM sleep before and after that point was roughly the same, whether the researchers studied rabbits, rats, pigs or humans.

数据非常一致：当所有物种达到相当于人类大约2岁半的发育年龄时，它们快速眼动睡眠的时间都急剧下降。无论研究对象是兔子、老鼠、猪还是人类，在这之前和之后的快速眼动睡眠时间比例大致相同。

REM sleep decreases with the growth in brain size throughout development, the scientists found. While newborns spend about 50% of their sleep time in REM sleep, that falls to about 25% by the age of 10 and continues to decrease with age. Adults older than 50 spend approximately 15% of their time asleep in REM. The significant dropoff in REM sleep at about 2-and-a-half happens just as the major change in the function of sleep occurs, Poe said.

科学家们发现，在大脑发育过程中，快速眼动睡眠会随着大脑体积的增大而减少。新生儿50%的睡眠时间是快速眼动睡眠，10岁时降至25%，并随着年龄的增长继续下降。50岁以上的成年人大约有15%的时间处于快速眼动睡眠状态。Poe说，快速眼动睡眠的显著下降大约发生在2岁半时，此时睡眠功能发生了重要变化。

“Sleep is as important as food,” Poe said. “And it’s miraculous how well sleep matches the needs of our nervous system. From jellyfish to birds to whales, everyone sleeps. While we sleep, our brains are not resting.”

“睡眠和食物一样重要，”Poe说。“不可思议的是，睡眠与我们神经系统的需求是如此匹配。从水母到鸟类到鲸鱼，每个人都睡觉。当我们睡觉时，我们的大脑并没有休息。”

A chronic lack of sleep likely contributes to long-term health problems such as dementia and other cognitive disorders, diabetes, and obesity, to name a few, Poe said. When you start to feel tired, she said, don’t fight it — go to bed.

Poe说，长期睡眠不足可能会导致长期的健康问题，如痴呆和其他认知障碍、糖尿病和肥胖等。她说，当你开始感到累的时候，不要和倦意斗争——去睡觉。

“I fought sleep and pulled all-nighters when I was in college, and now think that was a mistake,” Savage said. “I would have been better off with a good night’s sleep. Now when I feel tired, I don’t have any guilt about sleeping.”

Savage说:“上大学的时候，我经常熬夜，现在看来这真是个错误。如果能睡个好觉，我状态就会更好。现在当我感到累的时候，我一点也不觉得睡觉有什么罪恶感了。”

For most adults, a regular seven-and-a-half hours of sleep a night is normal — and time lying awake doesn’t count, Poe says. While children need more sleep, babies need much more, roughly twice as much as adults. The large percentage of REM sleep in babies is in stark contrast to the amount of REM sleep observed in adult mammals across an enormous range of brain sizes and body sizes. Adult humans have five REM cycles during a full night of sleep and can have a few dreams in each cycle.

Poe说，对于大多数成年人来说，每晚有规律地睡7个半小时是正常的，但醒着躺着的时间不算在内。同时儿童需要比成人多的睡眠，并且婴儿需要的睡眠更多，大约是成年人的两倍。婴儿在快速眼动睡眠中所占的很大比例与成年哺乳动物的快速眼动睡眠形成了鲜明的对比，尽管成年哺乳动物的大脑和身体尺寸都有很大的变化。在一整夜的睡眠中，成年人有五个快速眼动周期，每个周期可以做几个梦。

A good night’s sleep is excellent medicine, Poe says. And it’s free.

Poe说，晚上睡个好觉是极好的良药。而且它是免费的。