Sleep Duration Is Again Linked to Mortality

By Dr. Gregg D. Jacobs

An editorial entitled "Do We Sleep Too Much?" by Dr. Daniel Kripke of the University of California at San Diego was published in the current issue of the journal Sleep. In this editorial, Kripke comments on a study in this issue of Sleep by Dr. A. Tamakoshi and colleagues in Japan that showed that sleep duration is linked to mortality. The Tamakoshi study found that the best survival is experienced by those who slept 6.5-7.5 hours on weekdays, and that the mortality risk is greater for those who sleep more than 7.5 hours than those who sleep less than 6.5 hours. Sleeping 8 hours per night was associated with greater mortality risk than sleeping 7 hours; increasing sleep durations of 9 or 10 hours were associated with increasing mortality risk; and, sleeping 5-6 hours per week night was not associated with significantly greater mortality than sleeping 7 hours. Only those who reported sleeping 4 hours or less experience significantly increased mortality. Mental stress, depression, and sleep apnea did not appear to be the cause of the relationship between sleep duration and mortality.

These findings are fully consistent with several recent studies, including the Nurses Health Study and the Cancer Prevention Study that found the same association between sleep duration and mortality. All thee studies are in agreement that sleeping more than 7.5 hours per night is associated with greater mortality than sleeping less than 6.5 hours per night, and that long sleep is associated with greater mortality than short sleep. The findings of these three studies involving enormous samples (up to one million people) are consistent with a dozen smaller studies, and no persuasive studies have contradicted these findings. Thus, the scientific evidence appears incontrovertible that that the widely-circulated recommendation that it is best to sleep at last eight hours is false.

The cause of increased mortality associated with eight or more hours of sleep remains unclear, and it is unclear of long sleepers should curtail their sleep.