How much time should one sleep!!!

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 hours and 7.5 hours a night, as they report, live the longest. And people who sleep 8 hours or more, or less than 6.5 hours, 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 hours. Sleeping 8.5 hours might really be a little worse than sleeping five.

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 seven or eight hours, but there are some at six and some even at nine. I think diabetes is lowest in seven-hour sleepers, [for example]. But these measures aren't as clear as the mortality data.

I think we can speculate [about why people who sleep 6.5 to 7.5 hours 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 OK. We've all been told you ought to sleep eight hours, 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 eight hours or nine hours, so they spend eight or nine hours in bed, with the result that they have trouble falling asleep and they 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 United States, 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 they 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.