People who sleep for seven hours a night live longestBMJ 2002; 324 doi: http://dx.doi.org/10.1136/bmj.324.7335.446/e (Published 23 February 2002) Cite this as: BMJ 2002;324:446
A study of the sleep habits of more than one million people over six years seems to debunk the popular idea that eight hours of sleep nightly are required for optimal health. Those who had six or seven hours had a lower death rate than those who regularly slept eight or more hours—or less than four.
The study included adults aged 30 to 102 years and was carried out by researchers from the University of California at San Diego School of Medicine and the American Cancer Society. It was conducted from 1982 to 1988, but results became available only recently because of the time required to input and analyse the vast amount and variety of data from the 1.1 million participants (Archives of General Psychiatry 2002:59:134-6).
The study did not explain why there was an association between longer sleep and higher mortality.
The first author, Dr Daniel Kripke, a professor of psychiatry at the university who specialises in sleep research, said: “We don't know if long sleep periods lead to death. Additional studies are needed to determine if setting your alarm clock earlier will actually improve your health. [But] individuals who now average 6.5 hours of sleep a night can be reassured that this is a safe amount of sleep. From a health standpoint, there is no reason to sleep longer.”
The research was conducted as part of the second cancer prevention study of the American Cancer Society. Participants who reported occasional bouts of insomnia did not have an increased death rate, but those who took sleeping pills were more likely to die sooner.
“Insomnia is not synonymous with short sleep,” said the study. “Patients commonly complain of insomnia when their sleep durations are well within the range of [those of] people without sleep symptoms.”
This was the first large scale population study of sleep to compare individuals with similar characteristics such as age, diet, exercise, previous health problems, and risk factors such as smoking. Dr Kripke said that previous sleep studies have indicated that both long and short duration sleep carried with them higher mortality, but no study was large enough to distinguish the difference between seven and eight hours a night.
Best survival rates were found among those who slept seven hours a night. A group sleeping eight hours was 12% more likely to die within the six year period than those sleeping seven hours, other factors being equal. Even those with as little as five hours lived longer than those with eight hours or more nightly.
Mean ages of those in the study were 57 for women and 58 for men. Within the six year period, 5.1% of the women died and 9.4% of the men. Causes of death resembled the distribution of those of the general population.