To our knowledge, this is the first quantification of the amount of time spent in overall sedentary behaviors in a large representative population sample in the United States. Time spent in these behaviors reflects the accumulated amount of time spent sitting, reclining, or lying down at home, at work, at school, in transit, and during leisure time. These US children and adults spent approximately 55 percent of their waking hours, or 7.7 hours/day, in behaviors that result in only very low levels of energy expenditure. Older adolescents (ages 16–19 years) and older adults (ages 60–85 years) spent nearly 60 percent of their time, or more than 8 hours/day, in sedentary behaviors. In contrast, children aged 6–11 years spent only 6 hours/day in these behaviors. Mexican-American adults were the least sedentary group of adults, while Black and White adults tended to be similarly sedentary. Given the amount of time spent in these behaviors on a daily basis, particularly for older adolescents and adults, reducing the amount of time spent being sedentary represents an important opportunity for increasing the level of physical activity in the population.
Our study differs significantly from other population-based estimates of sedentary behaviors, in terms of both methodology and critical findings, and thus provides unique insight into the current level of sedentary behavior in the United States. In contrast to previous estimates of sedentary behavior derived from self-reports of a limited number of behaviors (i.e., television viewing and computer use), we directly quantified the total amount of time spent at very low activity levels over an average of 13.9 hours/day. Given our method of estimating time spent in sedentary behavior, it is not surprising that our results showed that children and adults are twice as sedentary as indicated by previous estimates derived only from assessment of media-related discretionary behaviors (13
). These results highlight the idea that media use accounts for only about half of the overall time spent in sedentary behavior in the US population.
The second important finding from our study is that previously quantified differences in television viewing and computer use that have been described by age, gender, and ethnicity may not translate to differences in levels of overall sedentary behavior. For example, prevalence estimates of sedentary behavior, defined as spending 3 or more hours per day watching television or using a computer, from the Youth Risk Behavior Surveillance System in 2005 indicated that males were more sedentary than females, Blacks were more sedentary than Hispanics or Whites, and time spent in these behaviors decreased between grades 9 and 11 (20
). Our study revealed a markedly different picture among youth. We found that girls were more sedentary than boys, Blacks were similar to both Whites and Mexican Americans, and the level of overall sedentary behavior increased substantially during adolescence. Perhaps the most striking variation in our results from previous studies was seen among Black and White children. Previous reports have found that approximately 40 percent of Black children reported watching 4 or more hours of television per day, while only about 20 percent of White children reported this much television viewing (21
). We found no evidence that this difference in television viewing translated into differences in objectively determined levels of overall sedentary behavior between White and Black children.
Close evaluation of time spent in sedentary behaviors across the wide age range in these data revealed two dramatic peaks that merit comment. Over the approximate decade from ages 6–11 years to ages 16–19 years, there was about a 2-hour/day increase in sedentary behavior, and this was approximately the same magnitude of increase as that observed across the four decades from young adulthood (ages 20–39 years) to older age (ages 70–85 years). Previous studies have also reported that the rate of decline in physical activity levels is greater in adolescence than in adulthood (23
). Studies using objective measures of sedentary behavior to describe age effects in youth have reported results that are similar to ours (24
). For example, Treuth et al. (24
) described nearly a 2-hour/day increase in time spent in sedentary behaviors when comparing elementary, middle, and high school students (proportion of monitored time in sedentary behavior: 46.7 percent, 51.4 percent, and 56.0 percent, respectively), and this increase was predominantly due to a reduction in time spent in light-intensity activities (i.e., 100–899 counts/minute by Actigraph). Pate et al. (26
) recently reported that sixth-grade girls (ages 11–12 years) spent 55.6 percent of their time, or 7.7 hours/day, in sedentary behaviors, which is virtually identical to results from the present study. Fewer studies have estimated the amount of time adults spend in sedentary behaviors, but results from the American Time Use Survey indicate that a large proportion of the average 15.4-hour waking day is spent in activities that expend very little energy (13
). In concordance with our study, Healy et al. (16
) employed the Actigraph and reported that Australian adults (average age 53.3 years) spent 57 percent of their monitored time in activities below 100 counts/minute. The reduction in time spent being sedentary between adolescence (ages 16–19 years) and early adulthood (ages 20–39 years) may reflect entrance into the workforce and domestic responsibilities related to raising families (13
). The increase in time spent in sedentary behaviors after age 60 years, particularly among men, may reflect an increase in leisure time following retirement and/or the development of comorbid health conditions that may increasingly limit activity in later life. Troiano et al. (27
) reported a reduction in overall activity levels with increasing age among men in this NHANES sample, which suggests that the increase in time spent in sedentary behavior we observed may not be offset by increases in leisuretime physical activity as reported by previous investigators (28
Time spent in pursuits that require only low levels of energy expenditure have been linked to increased risk of weight gain (2
), an adverse metabolic profile (4
), and type 2 diabetes (3
). Westerterp (1
) has described the importance of lower-intensity (e.g., light-to-moderate) activity on physical activity energy expenditure levels, and Levine et al. (2
) suggested that an additional 2 hours/day spent sitting conserved approximately 350 kcal/day in a small sample of obese adults, relative to lean adults. In the context of the current study, where we observed two age-related increases in sedentary behaviors of approximately 2 hours/day in adolescence and adulthood, we estimate that approximately 2 metabolic equivalent (MET)-hours/day (or 2 kcal/kg/day) of energy would be conserved by shifting 2 hours/day from light-intensity behaviors (2.5 METs) to sedentary behaviors (1.5 METs). For a person with a resting energy expenditure of 67 kcal/hour, this would equate to 134 kcal/day. This amount of energy is slightly greater than that associated with 30 minutes/day of brisk walking (0.5 hours/ day × 3.5 METs = 1.75 MET-hours/day, or 117 kcal/day) and is similar to the level of energy imbalance that has been associated with the current obesity epidemic (33
). Hence, even modest reductions in time spent in sedentary behaviors have the potential to increase energy expenditure and alter energy balance, if the increase in expenditure is not offset by a compensatory increase in energy intake.
Strengths of this study include the use of a large representative sample of children and adults in the United States, oversampling of Blacks and Mexican Americans to improve precision of the estimates in these groups, and the use of an objective measurement of overall sedentary behaviors. We supported our accelerometer cutpoint selection with a validation study in adults using a near-gold standard. This same cutpoint was identified in a calibration study among adolescent girls (15
). An additional strength of our study was that the majority of participants were quite compliant with the measurement procedures. While the minimum number of days of observation required to be included in our analyses was only 1, nearly 80 percent of the sample wore the device for at least 10 hours/day on 3 or more days. There has been some debate regarding the minimal number of days of observation required to achieve useful estimates from accelerometers in field-based studies (34
) and issues associated with missing data (36
). Our decision to use data for persons with at least one day of observation differs from decisions made by other investigators, but our results are virtually identical to those of Healy et al. (16
), who required at least 5 days of observation, including one weekend day, and those of Pate et al. (26
), who employed a minimum of 6 days of measured and imputed observation. This suggests that our methods provide useful estimates of the population mean for amount of time spent in sedentary behavior.
Limitations of our study should also be considered. The amount of monitor-wearing time in our study (13.9 hours/ day) was approximately 1.5 hours/day less than the average waking time reported in other national surveys (13
). Thus, our estimates of time spent in sedentary behaviors are likely to have underestimated the actual amount of time spent in these behaviors. If we normalized the population average for the proportion of monitored time spent being sedentary (54.9 percent) to a 15.4-hour waking day, our estimate of time spent in sedentary behavior would increase from 7.7 hours/day to 8.5 hours/day. In addition, the assessment of sedentary behaviors by accelerometer is relatively new, and our estimates of time spent in these behaviors will have been sensitive to the cutpoints employed. That said, it is unlikely that cutpoint selection would have affected our results with respect to age, gender, and race/ethnicity. Future studies that use more direct estimates of body position (e.g., see the paper by Grant et al. (37
)) are needed to confirm the present results. Additionally, our results provide estimates of the average amount of time spent per day in sedentary behaviors and reflect the influence of both weekend days and weekdays; patterns of behavior may vary on these days. Finally, our cross-sectional study design limits causal inferences, particularly for the age differences reported, and our results are generalizable only to the noninstitutionalized population of the United States. Results from this report should be interpreted accordingly.
In conclusion, children and adults in the United States spend the majority of their waking time in sedentary behaviors. The most sedentary groups in the population are older adolescents (ages 16–19 years) and older adults (ages ≥60 years), while the least sedentary groups are children (ages 6–11 years) and Mexican-American males. Given the large amount of time spent each day in sedentary behaviors in the United States, efforts to reduce the amount of time spent in low-energy-expenditure pursuits are warranted.