U-Shaped Curve for Sleep Duration and Cardiovascular Disease

From Heartwire

Sue Hughes

February 10, 2011 (Warwick, United Kingdom) — Both short and long duration of sleep are predictors or markers of cardiovascular outcomes, a new review suggests [1].

The review, published online February 7, 2011 in the European Heart Journal, included 15 prospective studies in a total of 474 684 participants and shows an increased risk of developing or dying of CHD and stroke on either end of the distribution of sleep duration. But results for total cardiovascular events showed no detectable effect in short sleepers and a significant increased risk in long sleepers.

In the studies, sleep duration was assessed by questionnaire and incident cases through certification and event registers. Follow-up ranged from 6.9 to 25 years, and there were a total of 16 067 events.
Normal sleep time was defined as seven to eight hours per night.
Short sleepers were designated as those with less than five to sixhours per night, and long sleepers as those with more than eight to nine hours per night.

The researchers, led by Dr Francesco Cappuccio (Warwick Medical School, Coventry, UK), say that the mechanisms behind these associations are not fully understood. They list causative mechanisms relating short duration of sleep to adverse health outcomes as including changes in circulating levels of leptin and ghrelin, which in turn would increase appetite, caloric intake, reduce energy expenditure, and facilitate the development of obesity and impaired glycemic control, with increased cardiovascular risk. Increased cortisol secretion and altered growth hormone metabolism have also been implicated. Low-grade inflammation is also activated during short sleep, with possible implications not only for cardiovascular disease but also for other chronic conditions, including cancer.

Long Sleep: Just a Marker of Comorbidity

However, they note that there have been no postulated mechanisms to explain long duration of sleep as a cause of cardiovascular disease, and this association may just be a result of residual confounding and comorbidities. In particular, depressive symptoms, low socioeconomic status, unemployment, low level of physical activity, and undiagnosed health conditions have all been shown to be associated with long duration of sleep and to confound the association with morbidity and mortality. They add that a recent intervention study of weight reduction, healthy diet, and increased physical activity showed, compared with a control group, a significant reduction in the seven-year incidence of type 2 diabetes among long sleepers, supporting the view that long sleep may be an indicator of risk, reversible upon changes in the risk factors.

They conclude that people reporting consistently sleeping five hours or less per night should be regarded as a higher-risk group for cardiovascular morbidity and mortality. And that sleeping nine hours or more per night may represent a useful diagnostic tool for detecting subclinical or undiagnosed comorbidity.

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