Original ArticleSelf-reported sleep duration, all-cause mortality, cardiovascular mortality and morbidity in Finland
Introduction
The original observational finding of the U-shaped association between self-reported sleep duration and all-cause mortality [1], [2] was later confirmed in at least 12 studies [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] with only two studies failing to confirm the association [15], [16]. The first meta-analysis [17] also supported a U-shaped association between sleep duration and all-cause mortality.
Studies examining the relationship between sleep duration and specific death cause or ill health, like cardiovascular disease (CVD), have yielded less consistent results. CVD mortality and/or morbidity has been predicted by long sleep duration in five studies [5], [18], [19], [20], [21]; three studies found only short sleep as an independent predictor [22], [23], [24], two studies found an independent U-shaped association [25], [26], one study found a U-shaped association in men but only long sleep duration to be independently associated with CVD mortality in women [27], and two studies failed to confirm an independent association between sleep duration and CVD mortality [15], [28].
The nature and mechanisms yielding the association between sleep characteristics and mortality risks are far from clear. Sleep duration may be a direct or indirect causal risk factor/predictor or merely a correlate of processes implicated in mortality. The independent statistical role of short sleep duration as a predictor of mortality or health risk has been more often decreased or abolished by controlling for pertinent risk factors when compared with the role of long sleep duration [23], [29].
The factors predicting or influencing an individual’s sleep duration may vary, at least in quantity, between different countries or cultures [30], [31]. This may modulate the association between sleep characteristics and health risks across different countries. For example, in the United States sleep duration may have decreased during the last decades more strikingly [32] than in Finland, at least at the level of the adult general population [33]. Therefore, replications of the results on sleep and health risks within the same society are needed.
We wanted to clarify the associations of self-reported sleep duration with all-cause mortality and with cardiovascular mortality and morbidity in Finland during 1972–2006 using a representative population-based sample.
Section snippets
Participants
In two cross-sectional population surveys in the province of North Karelia and Kuopio, Finland, in 1972 and 1977 the levels of coronary risk factors and habitual sleep duration were measured. For both surveys an independent 6.6% random sample of 26,389 people born during the period 1913–1947 was drawn from the population register. Thus, the age range of the 1972 sample was 25–59 years, and the 1977 sample 30–64 years [34]. In all, 24,025 individuals (11,714 men and 12,311 women) participated in
Results
Baseline characteristics of the participants across self-reported night sleep duration are given in Table 1. In both genders there was a statistically significant U-shaped association between sleep duration and the following factors: age, total cholesterol, and systolic blood pressure. There was a U-shaped association of BMI and triglycerides with sleep duration only in women. The statistically significant association of smoking with sleep duration, however, was not U-shaped in women. In women
Discussion
The main results of this study yielded by a relatively large cohort representative of the Finnish adult general population with a long (29–34 years) follow-up period showed that CVD mortality and morbidity were associated with sleep duration. Increased unadjusted CVD risk was found in both short sleeper groups (extremely short ⩽5 h, and 6 h sleepers) as well as in extremely long sleepers (⩾10 h) in both genders when compared with mid-range (7–8 h) sleepers. The association, however, was found to be
Conflict of interest
The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: doi:10.1016/j.sleep.2010.10.006.
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