We all know that we don't get enough sleep. But how much sleep do we really need?

我们都知道自己睡眠不足。但是我们真正需要多少睡眠呢？

Until about 15 years ago, one common theory was that if you slept at least four or five hours a night, your cognitive¹ performance remained intact; your body simply adapted to less sleep.

直到大约15年前，有这样一个理论：如果你每晚至少睡4或5小时，你的认知表现依然保持完好；而你的身体只是适应了较少的睡眠。

But that idea was based on studies in which researchers sent sleepy subjects home during the day — where they may have sneaked² in naps and downed coffee.

但是那种观点是基于这样研究：研究人员在白天把困倦的受试者送回家，而后者有可能在家里偷偷地小睡或者猛灌咖啡。

Enter David Dinges, the head of the Sleep and Chronobiology Laboratory at the Hospital at University of Pennsylvania, who has the distinction of depriving more people of sleep than perhaps anyone in the world.

宾夕法尼亚大学医院的睡眠与时间生物学实验室的负责人大卫·丁格斯剥夺过很多人的睡眠，就人数而言世界上无人能比。

In what was the longest sleep-restriction study of its kind, Dinges and his lead author, Hans Van Dongen, assigned dozens of subjects to three different groups for their 2003 study: some slept four hours, others six hours and others, for the lucky control group, eight hours — for two weeks in the lab.

为了2003年的研究，丁格斯和第一作者汉斯.凡东恩开展了一项同类研究中最长的睡眠限制研究，他们把受试者分成三组：一些人睡4小时，另一些人睡6小时，其他人则被幸运地分进了对照组，睡8小时——他们在实验室里待了两个星期。

Every two hours during the day, the researchers tested the subjects' ability to sustain attention with what's known as the psychomotor vigilance task, or P.V.T., considered a gold standard of sleepiness measures.

白天，研究人员每隔两小时就会使用精神运动警觉性任务来测试受试者持续集中注意力的能力。PVT被视为衡量困倦程度的黄金标准。

During the P.V.T., the men and women sat in front of computer screens for 10-minute periods, pressing the space bar as soon as they saw a flash of numbers at random³ intervals⁴.

参与PVT的男女需在电脑屏幕前坐10分钟，每当看到不定时闪现的数字时，就要立刻按下空格键。

Even a half-second response delay suggests a lapse⁵ into sleepiness, known as a microsleep.

就算只滞后半秒，也暗示着受试者陷入了睡眠之中，即微睡眠。

The P.V.T. is tedious but simple if you've been sleeping well. It measures the sustained attention that is vital for pilots, truck drivers, astronauts.

如果你的睡眠一直都很好，那么PVT就是一项乏味却简单的任务。它衡量的是对飞行员、卡车司机和宇航员来说至关重要的持续性注意力。

Attention is also key for focusing during long meetings; for reading a paragraph just once, instead of five times; for driving a car. It takes the equivalent of only a two-second lapse for a driver to veer⁶ into oncoming traffic.

在参加冗长的会议时、在一次性阅读一个段落，而非看五遍时，以及开车时，注意力同样是保持专注的关键所在。一名司机大约只要走神两秒钟，就有可能冲入迎面而来的车流中。

Not surprisingly, those who had eight hours of sleep hardly had any attention lapses⁷ and no cognitive declines over the 14 days of the study. What was interesting was that those in the four- and six-hour groups had P.V.T. results that declined steadily⁸ with almost each passing day.

在为期14天的研究中，每天睡8小时的人几乎没走过神，也没有出现认知能力下降的问题，这并不令人意外。有趣的是，睡4小时和6小时的那些人的PVT测试成绩几乎逐日稳步下降。

Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task.

虽然睡4小时的受试者的表现要差得多，但是睡6小时的受试者也常常分心。

By the sixth day, 25 percent of the six-hour group was falling asleep at the computer. And at the end of the study, they were lapsing⁹ fives times as much as they did the first day.

到了第六天，睡6小时的那组人中有25%会在电脑前睡着。而在研究的收尾阶段，他们走神的次数达到第一天的5倍。

The six-hour subjects fared no better — steadily declining over the two weeks — on a test of working memory in which they had to remember numbers and symbols and substitute one for the other.

在对工作记忆的测试中，受试者必须记住一些数字和符号，并用一个代替另一个--睡6小时的受试者没有更好的表现--而是在两周之内稳步下降。

The same was true for an addition-subtraction task that measures speed and accuracy.

在测量速度和准确度的加减运算任务中，情况也是如此。

All told, by the end of two weeks, the six-hour sleepers¹¹ were as impaired¹² as those who, in another Dinges study, had been sleep-deprived for 24 hours straight — the cognitive equivalent of being legally drunk.

总之，两周结束时，睡6小时的受试者的能力被削弱了，就像丁格斯的另一项研究中整整24小时没有睡过觉的人一样--其认知能力跟那些在法律上会被认定为醉酒的人差不多。

Not every sleeper¹⁰ is the same, of course: Dinges has found that some people who need eight hours will immediately feel the wallop of one four-hour night, while other eight-hour sleepers can handle several four-hour nights before their performance deteriorates¹⁴. (But deteriorate¹³ it will.)

当然，睡眠者的情况并非千篇一律：丁格斯发现，一些需要睡8小时的人如果有一晚只睡4小时，情况立刻就会变得很糟，而另外一些需要睡8小时的人在好几晚只睡4小时之后，表现才会变差。（但终究会变差。）

There is a small portion of the population — he estimates it at around 5 percent or even less — who, for what researchers think may be genetic¹⁵ reasons, can maintain their performance with five or fewer hours of sleep. (There is also a small percentage who require 9 or 10 hours.)

有一小部分人--据他估计约为5%，甚至更低--可以在只睡5小时，甚至更短时间的情况下维持其表现，研究人员认为这是遗传原因。（还有一小部分人需要睡9或10小时。）