Objectives Higher levels of positive affect and feelings of energy are associated with greater physical activity (PA) and lower sedentary time (ST). However, whether fluctuations in these feelings contribute to the regulation of these behaviors is unclear. This study examined the extent to which subject-level variability (i.e., degree of intraindividual variability) in positive affect and feeling energetic predicted participants’ overall levels of PA and ST. Design This analysis combined data from four ecological momentary assessment (EMA) studies (agerange: 8–73 years) with ambulatory monitoring via waist-worn accelerometry (N = 661). Methods Positive affect and energy were assessed through EMA several times per day across 4–7 days. Accelerometer data was used to create the following behavioral outcomes: (1) meeting MVPA guidelines (children: 60 minutes/day, adults: 30 minutes/day) and (2) minutes of ST per hour of accelerometer wear. A two-stage analytic approach was used to test the study aim. In the first stage, mixed-effects location scale modeling decomposed mean levels and variability in positive affect and energy. In the second stage, a linear or logistic regression (depending on whether the outcome was continuous or dichotomous, respectively) was tested to investigate associations between subject-level mean and variability in EMA ratings and the behavioral outcome. Results Greater subject-level variability but not subject-level mean of feeling energetic was associated with lower odds of meeting MVPA guidelines (β = −0.43, p < 0.05). Conclusions Fluctuations in physical feeling states may deplete self-regulatory resources involved in planning and implementing PA behavior. Alternatively, being more physically active may stabilize one’s perceived energy levels.