PURPOSE: Excessive sedentary time (SED) has been linked to obesity and other adverse health outcomes. However, few sedentary-reducing interventions exist and none have utilized smartphones to automate behavioral strategies to decrease SED. We tested a smartphone-based intervention to monitor and decrease SED in overweight/obese individuals, and compared 3 approaches to prompting physical activity (PA) breaks and delivering feedback on SED. DESIGN AND METHODS: Participants (n = 30; age 47.5 (13.5) years; 83% female; body mass index (BMI) 36.2 (7.5) kg/m2) wore the SenseWear Mini Armband (SWA) to objectively measure SED for 7 days at baseline. Participants were then presented with 3 smartphone-based PA break conditions in counterbalanced order: (1) 3-min break after 30 SED min; (2) 6-min break after 60 SED min; and (3) 12-min break after 120 SED min. Participants followed each condition for 7 days and wore the SWA throughout. RESULTS: All PA break conditions yielded significant decreases in SED and increases in light (LPA) and moderate-to-vigorous PA (MVPA) (p < 0.005). Average % SED at baseline (72.2%) decreased by 5.9%, 5.6%, and 3.3% (ie, by mean (95% CI) -47.2 (-66.3 to -28.2), -44.5 (-65.2 to -23.8), and -26.2 (-40.7 to -11.6) min/d) in the 3-, 6-, and 12-min conditions, respectively. Conversely, % LPA increased from 22.8% to 26.7%, 26.7%, and 24.7% (ie, by 31.0 (15.8 to 46.2), 31.0 (13.6 to 48.4), and 15.3 (3.9 to 26.8) min/d), and % MVPA increased from 5.0% to 7.0%, 6.7%, and 6.3% (ie, by 16.2 (8.5 to 24.0), 13.5 (6.3 to 20.6), and 10.8 (4.2 to 17.5) min/d) in the 3-, 6-, and 12-min conditions, respectively. Planned pairwise comparisons revealed the 3-min condition was superior to the 12-min condition in decreasing SED and increasing LPA (p < 0.05). CONCLUSION: The smartphone-based intervention significantly reduced SED. Prompting frequent short activity breaks may be the most effective way to decrease SED and increase PA in overweight/obese individuals. Future investigations should determine whether these SED reductions can be maintained long-term. TRIAL REGISTRATION: ClinicalTrials.gov NCT01688804.

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