“The surprise was that when sleep was
measured objectively, the best
survival was observed among women who
slept 5 to 6.5 hours,” Kripke said.
“Women who slept less than five hours
a night or more than 6.5 hours were
less likely to be alive at the 14-year
We'll have to see the full study to understand what they mean by "measured objectively" ... I get about 6 hours per night and feel very energetic throughout the day and actually find these results to be encouraging, although it is specific to women at this point.
The title of the original article is "Mortality related to actigraphic long and short sleep."
Here are a few points:
The results are based on data collected from 1995-1999. "Actual sleep" was determined by an actigraph, sort of like a pedometer worn on the wrist.
The results are based on estimated survival, not actual morality rates, because many women could not be contacted in the follow-up.
From the article, "Subsample recruitment was deliberately structured to include as many older women as possible to increase the power of mortality analyses."
The age range of the women in the study was 50-81. We know that older people sleep less than younger people.
The authors do not make causal claims in the original article. It is typical for the popular media to spin it that way, though.
Unfortunately the data are not fully consistent with their predicted u-shaped conclusion, because the average estimated survival rates for those women who slept less than 4 hours and also for those who slept more than 7.5 hours are nearly the same (86%) as those who slept 6-6.5h (90%). The lowest survival estimates were in the 4.5-5h (54%) and 7-7.5h (58%) sleep duration categories.
There is huge variability in the risk ratio of the two "worst" sleep duration categories: in the 7-7.5 and 4.5-5 hour rages. In my opinion, this helps explain why the results from these categories are so different from all the other categories.
Reported sleep is almost always longer than actual sleep, so the women who actually slept 6-6.5 hours probably reported about 7 hours of sleep.
This is fairly straightforwardly confusing cause and effect. Other studies have found that people who sleep less are healthier, but this isn't telling us whether healthier people need to sleep less, or sleeping less makes people healthier. Only the former looks particularly plausible.
It's analogous to the confusion over cholesterol: people who haven't damaged their bodies and thus require higher levels of cholesterol for repair will have lower levels of cholesterol, but this by no means implies that lowering the level of the substance that repairs said damage- cholesterol- will make you healthier.
Exactly as David said. SOunds like this is an epidiemiological study which means it can't determine cause. Sick people sleep more. Depressed people sleep more. People with busy lifestyles sleep less. Older people sleep less. You would have to separate out all those causes and more before you could even start to guess at any other causal relationships. Plus there was a 14 year gap between the data taken and the followup. Many of those people's sleep habits could have changed drastically in that time frame.
I will still stay with my usual advice which is sleep as much as your body seems to want on any given day. That's the natural way and lack of sleep stresses the body. Sleep needs are highly individual and definitely not an area where you will want to try force yourself into some kind of preset mold just because of the outcome of one shoddily done study. If you ask me, it was irresponsible of those researchers to suggest a specific sleep time based only on that flimsy data.
I am sleeping 6.5 hours per night max., working out 6-7 days per week(30-45 min. weights, supersetting sets, 30-40 min cardio twice per day, zero carbs, 3000 kcal.)---and I have never felt better! I used to sleep for up to 9 hours thinking it was healthy. Boy what a waste of time!
I know without any research and just from observation, that if I sleep less than 6.5 hours I feel like a zombie, with flu-like symptoms. I can do one day with 7h if I sleep more the next day. Generally I try to aim for 7.5-8h to feel good.
And I won't even mention that today I slept for 10h...(I have a day off!)
But we know very well that observation of one factor can lead to many mistaken conclusions. What if women who sleep 5-6.5h tend to also eat healthier?