U-shaped curve for sleep duration and cardiovascular disease

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.

Relative risk of developing or dying of CHD, stroke, or total CVD in short sleepers 

Outcome	RR (95% CI)	p
CHD	1.48 (1.22-1.80)	0.0001
Stroke	1.15 (1.00-1.31)	0.047
Total CVD	1.03 (0.93-1.15)	0.52

Relative risk of developing or dying of CHD, stroke, or total CVD in long sleepers 

Outcome	RR (95% CI)	p
CHD	1.38 (1.15-1.66)	0.0005
Stroke	1.65 (1.45-1.87)	0.0001
Total CVD	1.41 (1.19-1.68)	0.0001

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 six hours 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.