Moderation In All Things, Even Sleep
Abstract & Commentary
Synopsis: Women who habitually sleep 5 or fewer or 9 or more hours per night have an increased risk of coronary heart disease compared with those who sleep 7 or 8 hours a night.
Source: Ayas NT, et al. Arch Intern Med. 2003;163:205-209.
This study comes from the Nurse’s Health Study, an ongoing prospective observation of Registered Nurses in the United States. In 1986, 71,617 nurses who were 45-63 years old completed a survey of health habits. One question on the survey was: "Indicate total hours of actual sleep in a 24-hour period." Respondents were asked to choose 1 of the following options: 5 or fewer, 6, 7, 8, 9, 10, or 11 or more hours. Data were also collected about many relevant confounders, including age, smoking status, alcohol consumption, body mass index, menopausal status, hormone replacement, depressed mood (from the Short Form Health Survey), aspirin use, family history of cardiovascular disease, snoring, hypercholesterolemia, and duration of night shift work.
Sixty-five percent of the cohort slept 7 or 8 hours a day. Thirty-one percent slept 6 or fewer hours, and 5% slept 9 or more hours a day. Both increased and decreased sleep duration were associated with increased risk of diabetes, hypertension, and hypercholesterolemia. However, women in both the long-sleep and the short-sleep categories tended to be heavier. Women who slept longer were more likely to snore regularly, and women who slept less were more likely to report more shift working. After adjusting for the covariates, the Relative Risks (RR) for coronary heart disease were 1.45 for short sleepers (< 5 h/night) and 1.38 hours for long sleepers (> 9 h/night).
Comment by Barbara A. Phillips, MD, MSPH
These findings mirror those of Kripke et al,1 who reported last year (in a study of more than 1 million people!) that sleep times above 8 hours or below 6 hours are associated with increased mortality. In the current paper, Ayas and colleagues tend to focus on the "low end" of their U-shaped curve, that is that short sleepers had an increased risk of coronary heart disease in this study. They note that possible mechanisms include increased sympathetic activity,2 increased blood pressure,3 and glucose intolerance,2 which have been reported with sleep deprivation in previous studies. Given that about one-third of Americans sleep 6 or fewer hours a day,4 Ayas et al may have identified another prevalent modifiable risk factor for coronary heart disease. In fact, Dr. Ayas is quoted in USA Today as stating he has increased his sleep time from 6 hours to 7 or 8 a night.5 Of course, causality is not proven from this study; some behavioral or biological factors that they failed to control for (eg, stress) could be a causal factor both for reduced sleep and coronary heart disease. But these findings are consistent with other studies,1,6,7 and Ayas et al carefully controlled for many important confounders.
Ayas et al have tended to minimize the other side of the U; that is, the long sleep side. There was no statistically significant increase in coronary heart disease in women who slept 7 hours a night (though Ayas et al referenced everything to 8 hours a night), but those who slept 9 or more hours actually had a higher adjusted relative risk for coronary heart disease than those who slept 6 hours a night. Ayas et al profess "surprise" at this finding, and speculate that sleep apnea could be the mediator, even though there is no evidence that people with sleep apnea are long sleepers. They wind up concluding that increased sleep duration may be an early symptom of cardiac disease, which sounds a little lame to me. They discount as implausible the notion that increased sleep itself could lead to cardiovascular disease. However, sleep extension may result in increased rapid eye movement (REM) sleep, a vulnerable time for the heart, or may predispose to thromboembolism. At any rate, the finding that long sleepers have increased adverse events has been a consistent one on studies that have looked at both ends of the U,1,6,7 and it is just as scientifically valid as the association of short sleep with adverse outcome. The reason I am stressing this is because I see many insomniacs in my sleep medicine clinics. Although people with insomnia are a fairly disparate (and desperate!) group, they tend to have one thing in common: they overvalue sleep. Many believe that if they have a bad night’s sleep or if they don’t consistently get 8 hours of sleep, everything in their lives from job performance to marital satisfaction could suffer. Such people might benefit from learning that in this (and other studies), people seem to do pretty well with 6 or 7 hours of sleep. But not so well with 9 or more.
References
1. Kripke DF, et al. Arch Gen Psychiatry. 1979;36: 103-116.
2. Speigel K, et al. Lancet. 1999;23:1435-1439.
3. Tochibuko O, et al. Hypertension. 1996;27:1318-1324.
4. National Sleep Foundation. National Sleep Foundation Survey. Washington, DC. National Sleep Foundation; 2001.
5. Rubin R. Eight hours of sleep is optimum for women’s heart health, study finds. USA Today. January 28, 2003:8D.
6. Qureshi AI, et al. Neurology. 1997;48:904-911.
7. Wingard DB, et al. Sleep. 1983;6:102-107.
Dr. Phillips is Professor of Medicine at the University of Kentucky and Director of Sleep Disorders Center, Samaritan Hospital, Lexington, KY.
Women who habitually sleep 5 or fewer or 9 or more hours per night have an increased risk of coronary heart disease compared with those who sleep 7 or 8 hours a night.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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