Original ArticleMortality related to actigraphic long and short sleep
Introduction
People often wonder if they are getting enough sleep, reflecting the ancient folk belief that adults should sleep 8 h. In 1964, Hammond reported that among men participating in the Cancer Prevention Study I (CPSI), those who reported 7 hours of sleep had survived longer than those who slept 8 h or more [1], casting doubt on the 8-h belief. In addition, men who reported less than 7 h sleep had shorter survival. A subsequent analysis strengthened these findings using a more complete sample of the CPSI men and women [2]. More than two decades later, after the Cancer Prevention Study II (CPSII) was completed, an extensive analysis of the new sample of 1.1 million participants who were followed for 6 years, controlling for 32 covariates, continued to show that those with 6.5–7.4 h of sleep had lower mortality than those with shorter or longer self-reported sleep durations [3]. Somewhat to the surprise of many observers, this analysis showed that reported insomnia was not a mortality risk factor after control for confounding covariates, but reported sleeping pill use was associated with substantial increased risk after controlling for sleep duration, insomnia, and other covariates. Moreover, there was a greater proportion reporting long sleep >7.5 h than the proportion with short sleep <6.5 h, and a greater attributable mortality risk was associated with long sleep than with short sleep.
Mainly in the last decade, numerous studies have replicated the general findings from the Cancer Prevention Studies. A large representative sample from Japan showed a similar mortality minimum at approximately the same reported sleep duration and likewise showed the more impressive risk associated with the longer sleep durations [4]. A recent meta-analysis confirmed these findings among the increasing body of available self-report studies, estimating that the risk ratio associated with short sleep was 1.10 (1.06–1.15, 95% CI), whereas that associated with long sleep was 1.23 (1.17–1.30, 95% CI) [5].
Reported sleep is only loosely associated with objectively-recorded sleep duration, especially in the age group above 60 years in which most sleep-associated deaths are observed [6], [7]. Whether mortality associated with reported sleep durations effectively represents mortality associated with objective physiologic sleep is a lingering question [8]. A well designed polysomnographic study was unable to verify any association of sleep durations less than 6 h with excess mortality, but only 66 participants had died among 184 recorded [9]. Since long sleep latency and poor sleep efficiency were associated with decreased survival in that study, a new question emerged whether excessive time spent in bed was associated with excess mortality [9]. The larger issue is whether the associated mortality is caused or mediated by long or short sleep, or whether sleep duration is merely a comorbid marker associated with other yet-to-be-defined causal processes.
To determine if objective sleep duration is associated with mortality, from October 1995 through June 1999 we collected actigraphic sleep recordings of women who were participating in the Observational Study of the Women’s Health Initiative (WHI), University of California, San Diego Clinical Center. These observations were funded independently as an ancillary study of the WHI. Several previous analyses of the initial data from our participants have appeared, as described in Appendix 1, but the excellent survival of these women has required up to 14 years of follow-up to ascertain sufficient deaths so that the pre-planned mortality analyses could be performed.
Section snippets
Methods
As described in our previous publications in more detail, 459 women who were already participating in the Women’s Health Initiative as part of the Observational Study at the San Diego Clinical Center were recruited to participate in ancillary recordings of sleep. Subsample recruitment was deliberately structured to include as many older women as possible to increase the power of mortality analyses. Likewise, the subsample was structured to over-represent women subjectively reporting sleep
Results
Some usable data were obtained for 459 participants. The women’s ages at intake ranged from 50 to 81 years with a mean of 67.6 (SD 7.9) years. Of the 444 participants for whom follow-up vital status could be estimated, 86 deaths were recorded over a mean observation of 10.5 (SD 2.8) years before censoring. Thus, the utilized N for each analysis was never greater than 444 (96.7% of all participants) and was often reduced to about 350 (76.3%) in analyses which required adequate urine collections
Conclusions
These results confirmed our prospective hypothesis that objective actigraphic sleep would be associated with a U-shaped mortality risk, with both short sleep and long sleep associated with excess mortality. The mortality ratios associated with actigraphic short and long sleep were surprisingly large, as much as 2–4-fold. Moreover, in this sample, objectively measured sleep durations predicted significant mortality risk, whereas subjectively reported sleep durations did not, implying that the
Conflict of Interest
We state that there were no conflicts of interest.
Acknowledgement
Supported by NIH HL55983, HL071123, and NO1-WH-3-2120.
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