Background:Excessive sedentary time is ubiquitous in Western societies. Previous studies have relied on self-reporting to evaluate the total volume of sedentary time as a prognostic risk factor for mortality and have not examined whether the manner in which sedentary time is accrued (in short or long bouts) carries prognostic relevance.Objective:To examine the association between objectively measured sedentary behavior (its total volume and accrual in prolonged, uninterrupted bouts) and all-cause mortality.Design:Prospective cohort study.Setting:Contiguous United States.Participants:7985 black and white adults aged 45 years or older.Measurements:Sedentary time was measured using a hip-mounted accelerometer. Prolonged, uninterrupted sedentariness was expressed as mean sedentary bout length. Hazard ratios (HRs) were calculated comparing quartiles 2 through 4 to quartile 1 for each exposure (quartile cut points: 689.7, 746.5, and 799.4 min/d for total sedentary time; 7.7, 9.6, and 12.4 min/bout for sedentary bout duration) in models that included moderate to vigorous physical activity.Results:Over a median follow-up of 4.0 years, 340 participants died. In multivariable-adjusted models, greater total sedentary time (HR, 1.22 95% CI, 0.74 to 2.02]; HR, 1.61 CI, 0.99 to 2.63]; and HR, 2.63 CI, 1.60 to 4.30]; P for trend¬ < 0.001) and longer sedentary bout duration (HR, 1.03 CI, 0.67 to 1.60]; HR, 1.22 CI, 0.80 to 1.85]; and HR, 1.96 CI, 1.31 to 2.93]; P for trend¬ < 0.001) were both associated with a higher risk for all-cause mortality. Evaluation of their joint association showed that participants classified as high for both sedentary characteristics (high sedentary time ‚â•12.5 h/d] and high bout duration ‚â•10 min/bout]) had the greatest risk for death.Limitation:Participants may not be representative of the general U.S. population.Conclusion:Both the total volume of sedentary time and its accrual in prolonged, uninterrupted bouts are associated with all-cause mortality, suggestive that physical activity guidelines should target reducing and interrupting sedentary time to reduce risk for death.Primary Funding Source:National Institutes of Health.