In conclusion, this study is one of the first to investigate the role of subject-level variability in affect and arousal in regulating physical activity and sedentary time. Results indicate that variability in arousal is an important predictor of physical activity and adds to our theoretical understanding of the processes regulating health behaviors. These results add to accumulating evidence that researchers should move beyond focusing solely on mean levels and consider the extent to which people experience intraindividual or within-subject variability in relevant time-varying predictors to better understand health behaviors.