Sleep science is still progressing. It is possible that a dominant hypothesis turns to a fallacy tomorrow. At present, the best duration of sleep in a day is seven hours.
The Telegraph: Why regular lie-ins are a recipe for an early death
In this article, the new study is introduced whose result was suggesting that long-sleeper, more than eight hours in a day, tend to die early. The risk of oversleeping was equivalent to excessive taking of alcohol. Terrible!
This result is not surprising, to be honest. I have heard some similar studies indicating the harm of excessive sleep. It has been estimated that the ideal duration of sleep is between six to eight hours, accumulating evidence suggest.
However, as I always say in this kind of research, there are some room to believe this conclusion. First, most studies examining the sleep and health in human are observational. I have not seen any intervention studies. Thus, it is still unclear whether the attempt to reduce sleep time to seven hours a day would save your life. It is possible simply long-sleepers have some fatal impairments in their internal organs. If so, avoiding excessive sleep would no more beneficial for them, rather deteriorating their condition.
My past entry: Five essences for beautiful sleep
My past entry: Sleep science
Another disadvantage of non-controlled studies is confounding. There can be some factors relevant to both sleep-time and fatality. For example, some people have vulnerable to fatigue, requiring long sleep. In this case, long sleep may not a cause of early death. The researchers must have made an effort to get rid of the influence of confounders in the analysis stage. But it is impossible to identify all confounders in advance.
After all, it is a fact that seven-hour sleepers tend to survive, but unsure due to seven-hour sleep.
In my case, I noticed that I can be awake all the day after I took eight hours sleep. The next day after slept for seven hours, I feel sleepy after lunch, and take a nap. I am not sure which is healthier lifestyle for me. It is important what I should do, since I am not completely average person.
Anyway, sleep matter is still controversial.