Scientists have been debating about quantum of sleep for long time. But now a latest study has revealed that people having 6.5 to 7.5 hours of sleep live longest. But sleeping more or less doesn't help and they have a shorter life span. 

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.