32 Funnel plot asymmetry was used to detect publication bias, and Egger's regression test was applied to measure funnel plot asymmetry. The heterogeneity among studies was tested by Q-statistic and quantified by H-statistic and I 2-statistic. By comparison with the reference category of sleep duration, we estimated the pooled risk and 95% CI of death for the short and for the long sleep category, separately. The pooled RR (and 95% CI) was estimated using a random effect model. The value from each study and the corresponding standard error were transformed into their natural logarithms to stabilize the variances and to normalize the distribution. Their standard errors were calculated from the respective confidence intervals. Relative risks (RR) or hazard ratios (HR) were extracted from the selected publications and were used to measure the relationship between sleep duration and death. For the assessment of non-randomized, prospective studies, the maximum score is 19. It consists of several items distributed between 5 subscales: reporting, external validity, bias, confounding, and power. The quality of the studies included in the meta-analysis was evaluated by the Downs & Black Quality Index score system, 31 a validated checklist for assessing the quality of both randomized clinical trials and non-randomized studies. A meta-analysis allows the combination of the results of different studies, increasing the overall statistical power and the precision of estimates while controlling for bias and limiting random error. The aims of this study were to (a) systematically review published prospective population-based studies, (b) carry out a meta-analysis to assess whether the global evidence supports the presence of a relationship between either short or long duration of sleep and all-cause mortality, and (c) obtain a quantitative estimate of the risk to assess the consistency and potential public health relevance. The latter is important in public health to ascertain the likely impact at population level, if amenable to modification. 18 Given the variety of studies, the large differences in the types and sizes of populations examined, the duration of follow-up, and the size of the effects, it is difficult to draw immediate conclusions on the consistency of the associations at either end of the distribution of sleep duration and at its effect size. It is believed that different mechanisms may underlie such associations at either end of the distribution of sleep duration.
17 The relationship between duration of sleep and mortality has been often described as a U-shaped association, although other studies have not found such a uniform effect or have found no association. 5 Over the last few decades there has been growing evidence to suggest that too little sleep and too much sleep are associated with adverse health outcomes, including total mortality, 6, 7 cardiovascular disease, 6, 8 – 10 type 2 diabetes, 11 hypertension 12, 13 and respiratory disorders, 14 obesity in both children and adults, 15, 16 and poor self-rated health. 2 Lack of sleep also exerts deleterious effects on a variety of systems with detectable changes in metabolic, endocrine, 3, 4 and immune pathways.
1 This has led to increased reporting of fatigue, tiredness, and excessive daytime sleepiness. Sleep patterns of quantity and quality are affected by a variety of cultural, social, psychological, behavioral, pathophysiological, and environmental influences and have shown secular trends alongside changes in the modern society requiring longer hours of work, more shift-work and 24-7 availability of commodities, curtailing the duration of sleep to fewer hours per day across westernized populations. ITS underlying mechanisms, interactions, and long-term effects are still poorly understood. SLEEP IS AN ANCESTRAL AND PRIMITIVE BEHAVIOR THAT IS SHARED ACROSS THE PLANET BY OVER A BILLION PEOPLE ON A DAILY BASIS.
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