Sleep is one of the richest topics in science today: why we need it, why it can be hard to get, and how that affects everything from our athletic performance to our income. Daniel Kripke, co-director of research at the Scripps Clinic Sleep Center in La Jolla, Calif., has looked at the most important question of all. In 2002, he compared death rates among more than 1 million American adults who, as part of a study on cancer prevention, reported their average nightly amount of sleep. To many, his results were surprising, but they've since been corroborated by similar studies in Europe and East Asia. Kripke explains.

Q: How much sleep is ideal?

A: Studies show that people who sleep between 6.5 hr. and 7.5 hr. a night, as they report, live the longest. And people who sleep 8 hr. or more, or less than 6.5 hr., they don't live quite as long. There is just as much risk associated with sleeping too long as with sleeping too short. The big surprise is that long sleep seems to start at 8 hr. Sleeping 8.5 hr. might really be a little worse than sleeping 5 hr..

Morbidity [or sickness] is also "U-shaped" in the sense that both very short sleep and very long sleep are associated with many illnesses—with depression, with obesity—and therefore with heart disease—and so forth. But the [ideal amount of sleep] for different health measures isn't all in the same place. Most of the low points are at 7 or 8 hr., but there are some at 6 hr. and even at 9 hr. I think diabetes is lowest in 7-hr. sleepers [for example]. But these measures aren't as clear as the mortality data.

I think we can speculate [about why people who sleep from 6.5 to 7.5 hr. live longer], but we have to admit that we don't really understand the reasons. We don't really know yet what is cause and what is effect. So we don't know if a short sleeper can live longer by extending their sleep, and we don't know if a long sleeper can live longer by setting the alarm clock a bit earlier. We're hoping to organize tests of those questions.

One of the reasons I like to publicize these facts is that I think we can prevent a lot of insomnia and distress just by telling people that short sleep is O.K. We've all been told you ought to sleep 8 hr., but there was never any evidence. A very common problem we see at sleep clinics is people who spend too long in bed. They think they should sleep 8 or 9 hr., so they spend [that amount of time] in bed, with the result that they have trouble falling asleep and wake up a lot during the night. Oddly enough, a lot of the problem [of insomnia] is lying in bed awake, worrying about it. There have been many controlled studies in the U.S., Great Britain and other parts of Europe that show that an insomnia treatment that involves getting out of bed when you're not sleepy and restricting your time in bed actually helps people to sleep more. They get over their fear of the bed. They get over the worry, and become confident that when they go to bed, they will sleep. So spending less time in bed actually makes sleep better. It is in fact a more powerful and effective long-term treatment for insomnia than sleeping pills.

睡眠是当今科学领域的一个重要课题。我们为什么需要它？为什么会失眠？睡眠又是怎样影响我们日常的方方面面的？斯克里普斯诊所睡眠中心的研究员丹尼尔已经注意到了睡眠的一些关键性问题。2002年，丹尼尔对超过一百万美国人的死亡率做了对比研究——这些人会汇报他们晚上的平均睡眠时间——这个研究是癌症预防研究的一部分。对于该睡眠研究的结果，许多人会感到震惊，但事实上，这一结果已经被早前在欧洲和东亚的类似研究所证实。

问：睡多久才是合适的？

答：正如研究所表明的那样，晚上睡眠在6.5到7.5小时的人是最长寿的。而晚上睡眠超过8小时或不足6.5小时的人则要短命得多，这就是说，睡眠时间过长或者过短的人面临着同样的威胁。让人们感到惊讶的是，睡眠时间在8小时就可以称之为过长了。另外，8.5小时的睡眠时间比5小时要稍微糟糕些。

过多的睡眠和睡眠不足都与种种疾病相关，例如抑郁症、肥胖症、心脏病等，从这一意义上讲，疾病的发生率同样是成U字型的。但是，对于不同的健康要求，最适宜的睡眠时间是不一样的。大多数情况下，患者需要睡至少7或者8小时，但有的只需至少睡6小时，有的就要9小时。例如糖尿病患者就至少需要7小时的睡眠。但是，这一切并不像死亡率数据那么明显。

我认为，虽然我们可以推测出为什么每天睡6.5到7.5小时的人更加长寿，但我们必须承认，这其中的原因还是未知的，我们确实还不能找到原因，也不知道是什么在起作用。因此，如果一个睡眠不足的人增加睡眠时间，我们不知道他能不能因此而活得更久，我们也不知道一个睡眠过剩的人如果少睡一些，能不能活得更久。我们正准备着这方面的试验。

在我看来，我们可以告诉大家“睡眠时间短一点是完全没问题的”，这样可以减少许多人失眠和焦虑——这也是我发布这一研究成果的一个原因。我们总是告诫大家每天要睡8小时，但这是完全没有依据的。我们在睡眠诊所看到的一个普遍问题是，人们躺在床上的时间太长了。他们都认为每天应该睡8---9小时，所以他们就花那么多的时间躺在床上，结果是他们很难入睡，并且在夜间经常会醒来。许多失眠者会醒着躺在床上，担心着自己的失眠，这让我们感到很奇怪。欧美的许多控制失眠的研究都表明，失眠者应该不要躺在床上，减少在床上躺着的时间，这有利于获得更好的睡眠。失眠者要克服对床的恐惧，克服对失眠的忧虑，而且要对自己有信心，告诉自己：只要一躺在床上就能睡着。少点躺在床上吧，这确实能提高睡眠质量，这是对失眠者长期有效的治疗方法，功效比安眠药好得多