Eligible Studies
After searching prespecified public databases using predefined medical subject terms, a total of 2098 articles were initially identified, and 28 of them with data on sleep duration and all-cause mortality were eligible for inclusion [10, 14, 17, 19, 21, 25-47], including 95259 older persons in the final analysis. The detailed selection process including specific reasons for exclusion is schematized in Figure 1. Since most articles provided data according to different age groups at baseline or follow-up periods, they are processed separately in subgroup analyses.
Study Characteristics
Table 1 and Table 2 show the baseline characteristics of all cohort studies involved in this meta-analysis. Of 28 eligible articles, 2 in older women [17, 19], and 6 specifically described the number of men and women and the number of deaths of men and women [27, 30, 35, 38-40]. Five articles provided data on the association between sleep duration and all-cause mortality by gender [30, 35, 36, 38, 42]. Of all eligible articles, 9 investigated the total sleep duration of 24 hours in the older people [19, 26, 31, 33, 38, 39, 41-43], and the others focused on the nighttime. One article adopted the actigraphy method to collect sleep time [43], and 2 articles simultaneously used actigraphy method and questionnaires [17, 47]. Based on geographic locations, all eligible articles were classified into America [14, 17, 19, 26, 32, 33, 40, 42], Europe [10, 21, 34, 37, 42-45], and Asia [27-31, 35, 36, 38, 39, 41].
Quality Assessment
Table 3 shows the quality assessment results by using the Newcastle-Ottawa Scale (NOS) tool for cohort studies, with the total scores (mean: 7.46, standard deviation: 0.74) ranging from 6 to 9 in this meta-analysis.
Overall Analyses
After pooling the results of all qualified prospective cohorts together (Table 4), unadjusted effect-size estimates for the association of the long (HR=1.43; 95% CI: 1.30-1.58; P <.001; I2=88.6%) and short (HR=1.15; 95% CI: 1.06-1.25; P <.001; I2=71.5%) sleep duration with all-cause mortality in the older people were remarkably significant. After adjusting for potential confounders, long sleep duration was significantly associated with an increased risk of all-cause mortality in the older people (HR=1.24; 95% CI: 1.16-1.33; P <.001), whereas only marginal significance was observed for short sleep duration (HR=1.04; 95% CI: 1.00-1.09; P =.033) (Table 4). In view of the striking differences before and after adjustment, the following analyses are based on adjusted effect-size estimates for the sake of relative accuracy.
Publication Bias
Figure 2 shows the Begg’s funnel plot to assess publication bias for the association of sleep duration with all-cause mortality, and only the plot of short sleep duration seemed symmetrical. As revealed by the Egger’s test, there was no evidence of publication bias for short sleep duration (P =.392), yet strong evidence of publication bias for long sleep duration (P =.020). Further filled funnel plots showed that there were 9 potentially missing studies due to publication bias to make the plot of long sleep duration symmetrical. After adjusting for these potentially missing studies, effect size estimates were still statistically significant for the association of long sleep duration with all-cause mortality (HR=1.15; 95% CI: 1.07-1.23, P <.001).
Subgroup Analyses
A series of prespecified subgroup analyses were conducted to account for possible causes of between-study heterogeneity for both short and long sleep duration in the older people (Table 4).
By gender, the association of long sleep duration with all-cause mortality was statistically significant in both women (HR=1.48; 95% CI: 1.18-1.86; P =.001) and men (HR=1.31; 95% CI: 1.10-1.58; P =.003) (Two-sample Z test P =.205). By contrast, with regard to short sleep duration, statistical significance was observed in men (HR=1.13; 95% CI: 1.04-1.24; P =.007), but not in women (HR=1.00; 95% CI: 0.85-1.18; P =.999) (Two-sample Z test P =.099).
By geographic locations, the association of long sleep duration with all-cause mortality was stronger in Asia (HR=1.41; 95% CI: 1.26-1.57; P <.001) than in Europe (HR=1.01; 95% CI: 0.93-1.09; P =.823) (Two-sample Z test P <.001) and America (HR=1.19; 95% CI: 1.07-1.31; P =.001) (Two-sample Z test P = .013). There was no observable difference for short sleep duration between Asia (HR=1.04; 95% CI: 0.96-1.12; P =.384) and Europe (HR=1.03; 95% CI: 0.93-1.14; P =.627).
By total sleep time, significance was only observed for the association of long sleep duration with all-cause mortality, and there was no material difference between the nighttime (HR=1.25; 95% CI: 1.13-1.38; P <.001) and the 24 h sleep duration (HR=1.25; 95% CI: 1.14-1.36; P <.001).
By ascertainment of sleep, for long sleep duration, the association was more evident for questionnaire survey (HR=1.26; 95% CI: 1.17-1.35; P <.001) than for actigraph survey (HR=0.83; 95% CI: 0.61-1.13; P =.233) (Two-sample Z test P =.004). Contrastingly, for short sleep duration, there was no detectable significance.
By the median value (7.5 years) of follow-up intervals, the association of long sleep duration with all-cause mortality was significant in both long (≥7.5 years) (HR=1.24; 95% CI: 1.14-1.34; P <.001) and short (<7.5 years) (HR=1.27; 95% CI: 1.12-1.45; P <.001) follow-up. As for short sleep duration, the association was only significant in studies with long follow-up intervals (HR=1.07; 95% CI: 1.02-1.12; P =.006).
By the median value (65 years) of baseline age, long sleep duration was significantly associated with all-cause mortality in both subgroups (≥65 years: HR=1.20; 95% CI: 1.11-1.30; P <.001, and <65 years: HR=1.38; 95% CI: 1.19-1.60; P <.001), and for short sleep duration, only marginal significance was observed for studies with median age <65 years (HR=1.21; 95% CI: 1.02-1.23; P =.018).
Dose-Response Analyses
In the dose-response analysis on short sleep duration, all-cause mortality increased with the decrease of sleep time (≤5 h: HR=1.06; 95% CI: 1.01-1.11; P =.014, ≤6 h: HR=1.05; 95% CI: 1.01-1.10; P =.031, and ≤7 h: HR=1.04; 95% CI: 1.00-1.09; P =.033) (Two-sample Z test P =.379 for ≤5 h vs. ≤6 h, and P =.379 for ≤6 h vs. ≤7 h) (Table 4). For long sleep duration, the trend was more evident (≥8 h: HR=1.24; 95% CI: 1.16-1.33; P <.001, ≥9 h: HR=1.31; 95% CI: 1.21-1.41; P <.001, and ≥10 h: HR=1.45; 95% CI: 1.24-1.70; P <.001) (Two-sample Z test P =.147 for ≥8 h vs. ≥9 h, and P =.128 for ≥9 h vs. ≥10 h) (Table 4 and Figure 3A).
In men, the risk associated with all-cause mortality was significant and increased with both shorter and longer sleep duration, and the increasing trend was more obvious for long sleep duration (Figure 3B). In women, the risk associated with all-cause mortality was nonsignificant for short sleep duration, yet it was significantly increased with longer sleep duration in a graded manner, which was steeper than men (Figure 3C).
In both genders, dose-response regression analyses, using log (effect-size estimates) as dependent variable and categorized sleep duration as independent variable, revealed that trend estimation was more obvious for long sleep duration (regression coefficient: 0.13; P <.001) than for short sleep duration (regression coefficient: 0.02; P =.046) (Figure 4). In men, the regression coefficient for tread estimation was 0.05 (P =.022) and 0.15 (P <.001) for short and long sleep duration, respectively, and the regression coefficient was separately 0.04 (P =.449) and 0.20 (P <.001) in women.