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.

睡眠是當今科學領域的一個重要課題。我們為什么需要它？為什么會失眠？睡眠又是怎樣影響我們日常的方方面面的？斯克里普斯診所睡眠中心的研究員丹尼爾已經注意到了睡眠的一些關鍵性問題。2002年，丹尼爾對超過一百萬美國人的死亡率做了對比研究——這些人會匯報他們晚上的平均睡眠時間——這個研究是癌癥預防研究的一部分。對于該睡眠研究的結果，許多人會感到震驚，但事實上，這一結果已經被早前在歐洲和東亞的類似研究所證實。

問：睡多久才是合適的？

答：正如研究所表明的那樣，晚上睡眠在6.5到7.5小時的人是最長壽的。而晚上睡眠超過8小時或不足6.5小時的人則要短命得多，這就是說，睡眠時間過長或者過短的人面臨著同樣的威脅。讓人們感到驚訝的是，睡眠時間在8小時就可以稱之為過長了。另外，8.5小時的睡眠時間比5小時要稍微糟糕些。

過多的睡眠和睡眠不足都與種種疾病相關，例如抑郁癥、肥胖癥、心臟病等，從這一意義上講，疾病的發生率同樣是成U字型的。但是，對于不同的健康要求，最適宜的睡眠時間是不一樣的。大多數情況下，患者需要睡至少7或者8小時，但有的只需至少睡6小時，有的就要9小時。例如糖尿病患者就至少需要7小時的睡眠。但是，這一切并不像死亡率數據那么明顯。

我認為，雖然我們可以推測出為什么每天睡6.5到7.5小時的人更加長壽，但我們必須承認，這其中的原因還是未知的，我們確實還不能找到原因，也不知道是什么在起作用。因此，如果一個睡眠不足的人增加睡眠時間，我們不知道他能不能因此而活得更久，我們也不知道一個睡眠過剩的人如果少睡一些，能不能活得更久。我們正準備著這方面的試驗。

在我看來，我們可以告訴大家“睡眠時間短一點是完全沒問題的”，這樣可以減少許多人失眠和焦慮——這也是我發布這一研究成果的一個原因。我們總是告誡大家每天要睡8小時，但這是完全沒有依據的。我們在睡眠診所看到的一個普遍問題是，人們躺在床上的時間太長了。他們都認為每天應該睡8---9小時，所以他們就花那么多的時間躺在床上，結果是他們很難入睡，并且在夜間經常會醒來。許多失眠者會醒著躺在床上，擔心著自己的失眠，這讓我們感到很奇怪。歐美的許多控制失眠的研究都表明，失眠者應該不要躺在床上，減少在床上躺著的時間，這有利于獲得更好的睡眠。失眠者要克服對床的恐懼，克服對失眠的憂慮，而且要對自己有信心，告訴自己：只要一躺在床上就能睡著。少點躺在床上吧，這確實能提高睡眠質量，這是對失眠者長期有效的治療方法，功效比安眠藥好得多