Like other studies, the present study found that U.S. adults spent considerable time watching television or videos or using a computer. However, the present study offers little support for the hypothesis that prolonged screen time may increase mortality from all-causes and diseases of the circulatory system. Furthermore, control for confounding may be a particularly challenging issue in studies of screen time and mortality.
Previous prospective studies have reported variable findings about the relationship between the amount of television watching and mortality [13–16]. In an analysis of data from the Aerobics Center Longitudinal Study, 7744 men were followed for 21 years during which time 377 men died from cardiovascular disease . The hours per week of screen time were divided into quartiles. After adjusting for age, smoking, alcohol use, family history of cardiovascular disease, body mass index, physical activity, and self-reported hypertension, diabetes, and hypercholesterolemia, screen time was not significantly related to mortality from cardiovascular disease (quartile 2: HR = 1.02, 95% CI = 0.74, 1.42; quartile 3: HR = 1.27, 95% CI = 0.90, 1.78; quartile 4: HR = 0.96, 95% CI = 0.68, 1.36; p for trend = 0.94).
In the Australian Diabetes, Obesity, and Lifestyle Study, 8800 adults aged ≥ 25 years were followed on average for 6.6 years during which time 284 participants died (87 from cardiovascular disease) . Compared to adults who watched television < 2 h/day, those watching 2- < 4 h/day, and 4 or more hours per day had a significantly increased risk for mortality from all-causes (HR = 1.13, 95% CI = 0.87, 1.36; HR = 1.46, 95% CI = 1.04, 2.05, respectively) and cardiovascular disease (HR = 1.19, 95% CI = 0.72, 1.99; HR = 1.80, 95% CI = 1.00, 3.25, respectively). The hazard ratios were adjusted for age, gender, education, smoking status, energy intake, alcohol use, diet quality index, waist circumference, hypertension, total cholesterol, high-density lipoprotein cholesterol, triglycerides, lipid-lowering medications, and glucose tolerance status.
In the European Prospective Investigation into Cancer and Nutrition Norfolk Study, 13,197 men and women with a mean age of 61.5 years were followed for a mean of 9.5 years and 1270 participants died (373 from cardiovascular disease) . After adjusting for age, gender, education, smoking status, alcohol use, medications for hypertension and dyslipidemia, history of diabetes, family history of CVD and cancer, and physical activity, the hazard ratio per hour/day of watching television was 1.05 (95% CI: 1.01, 1,09) for all-cause mortality and 1.08 (95% CI: 1.01, 1.16) per hour/day for mortality from cardiovascular disease.
Recently, a fourth prospective study using data from the Scottish Health Survey was published . During a mean follow-up of 4.3 years, 325 deaths (215 CV deaths) occurred among 4,512 men and women aged ≥ 35 years. Compared to participants who watched television, used a computer, or played video games < 2 h per day, increased risks for all-cause mortality and fatal and nonfatal diseases of the circulatory system were noted for those who did so for 2- < 4 h per day (all-cause mortality aHR = 1.14, 95% CI = 0.80-1.62; diseases of the circulatory system aHR = 2.23 95% CI = 1.31-3.80) and ≥ 4 h per day (all-cause mortality aHR = 1.48, 95% CI = 1.04-2.13; diseases of the circulatory system aHR = 2.25 95% CI = 1.30-3.89). The risk estimates were adjusted for age, gender, ethnicity, body mass index, smoking, social class, long-standing illness, marital status, diagnosed diabetes and hypertension, occupational physical activity, and physical activity.
Of the four studies, including the present study, that examined the associations between screen time and all-cause mortality, only the present study failed to produce a significant association. Nevertheless, the adjusted hazard ratio of 1.31 in the present study for participants reporting ≥ 5 h per day of screen time is not inconsistent with the published estimates of screen time discussed above. The reasons for the dissonant findings are not entirely clear. All four studies used self-reported information to assess screen time although the questions in the studies differed. The follow-up times of the other three studies ranged from 4.3 years to 9.5 years compared to 5.8 years in the present study. Death status in all studies was determined through linkages to vital statistics.
Of the five studies, including the present one, that explored the associations between screen time and cardiovascular outcomes, two studies including the present one failed to report significant associations . A meta-analysis of previous prospective studies of screen time and all-cause mortality and fatal or nonfatal cardiovascular disease estimated that the summary relative risk per 2 h of screen time per day was 1.13 (95% CI = 1.07, 1.18) for all-cause mortality and 1.15 (95% CI = 1.06, 1.23) for fatal or nonfatal cardiovascular disease . Adding the results from the present study to the data shown in the meta-analysis changes the fixed-effects summary estimate of relative risk per 2 h of screen time to 1.12 (95% CI: 1.07, 1.17; test for heterogeneity p = 0.741) and 1.14 (95% CI: 1.06, 1.22; test for heterogeneity p = 0.736), respectively. Thus, the totality of the current evidence continues to suggest that prolonged screen time poses a threat to health.
Several additional prospective studies examined the relationships between sedentary behavior, which included watching television in some studies, and mortality [24–26]. Although these three studies did not focus on screen time per se, they did find that excessive sedentary behavior, primarily in the form of sitting, showed a small increase in the risk of mortality from all-causes and cardiovascular disease.
Strengths of the study included the population-based sample that is representative of adults in the United States (excellent external validity), decent sample size, good response rates, and inclusion of a broad spectrum of potential confounding variables and of several potential cardiometabolic mediators that were objectively measured. However, these strengths deserve to be balanced against various limitations of the study. Like many other studies, the amount of screen time was self-reported. The questions used in the survey were not specifically validated for this study. However, self-reported screen time has been shown to correlate with other measures in expected ways and, in the present study, correlated as expected with a number of anthropometric and clinical variables. A review of the reliability and validity of self-reported television viewing and other sedentary behavior described acceptable reliability but variable validity .
Because the question or questions used to measure screen time constitute a critical aspect of the prospective studies, it is interesting to note that all prospective studies to date differ in their assessment of screen time. The Aerobics Center Longitudinal Study assessed average weekly time spent watching television ; the Australian Diabetes, Obesity, and Lifestyle Study assessed total time spent watching television during the previous 7 days excluding time that the television was turned on but was not being watched ; the European Prospective Investigation into Cancer and Nutrition Norfolk Study asked about time spent watching television on weekdays and weekends ; and the Scottish Health Survey assessed the time spent watching television, using a computer, or playing video games on weekdays and weekends .
Consequently, some of the heterogeneity in the findings of the four studies examining the relationship between screen time and all-cause mortality and five studies of screen time and cardiovascular mortality might be attributable to the differences in questionnaires used to assess screen time. First, three of the previous studies assessed only the time spent watching television or watching videos [13, 14, 16], whereas the third study, like the present one, assessed the time spent watching television, using a computer, or playing video games . However, one of the first three studies failed to produce a significant association between screen time and cardiovascular disease whereas of the two studies that included time spent watching television, using a computer, or playing video games, one study reported a significant association  and the present study did not. Thus, it seems unlikely that this aspect of the questions about screen time explains the variation among studies. Nevertheless, the health effects of prolonged television viewing, which has been related to other unhealthy lifestyle behaviors notably unhealthy dietary elements, conceivably differ from those of prolonged use of a computer or playing video games.
Second, one study that reported significant associations between screen time and mortality from all-causes and cardiovascular disease attempted to limit screen time to the time that participants specifically watched television and not to the time that the television was turned on . However, among the remaining four studies that did not involve this methodological twist, two studies produced positive findings for mortality from cardiovascular disease [15, 16], and two studies, including the present one, produced negative findings . This aspect of the study questionnaires does not appear to readily explain the different findings of the studies.
Third, some studies measured screen time separately on weekdays and weekend days [15, 16] although not all studies were clear about this aspect of the exposure assessment. In the present study, screen time was not separately assessed for days during the week and weekend.
In the present study, participants reported sizeable amounts of screen time. The Nielsen Company data show that Americans spend on average about 35 h per week watching television, 2 h of time shifted television, and 4 h on the internet . The self-reported data from the present study suggests that the most commonly reported and median reported screen time was around 2 h, an estimate that is lower than the 2.7 h reported in the 2010 American Time Use Survey. Thus, underreporting of the amount of screen time by the NHANES participants is a possibility. If participants tended to underreport the true amount of screen time, the hazard ratios in the present study may have been underreported if participants at increased risk for mortality were shifted into the referent group thus raising the baseline risk of the referent group.
With 5.8 years of follow-up time, the duration of the present study is at the shorter end of the distribution, which ranges from 4.3 to 21 years. The study of 4.3 years found a significant association, whereas the study of 21 years did not. Consequently, it is unclear whether the duration of a study affects the chances of observing a significant result. As in virtually all observational studies, the results may have changed had additional known or unknown confounders been included in the analyses.