The objective of this study was to investigate changes in sedentary and active behaviors when previously inactive adults start exercising in the morning or evening. One-hundred adults with overweight or obesity (BMI ≥ 25 kg/m2) were recruited for a 12-week intervention and randomized to one of three groups: (i) morning exercise (AMEx; 0600–0900); (ii) evening exercise (PMEx; 1600–1900); or (iii) waitlist control. AMEx and PMEx were prescribed self-paced aerobic exercise to achieve a weekly total of 250 min via a combination of supervised and unsupervised training. Sedentary and active behavior times were measured at baseline, mid- and post-intervention using the multimedia activity recall for children and adults. Time spent engaging in physical activity was significantly increased from baseline at both mid- (+ 14–22 min·day−1) and post-intervention (+ 12–19 min·day−1), for AMEx and PMEx. At 12-weeks, participants in both morning and evening exercise groups reported increased time spent Sleeping (+ 36 and + 20 min·day−1, respecitively), and reduced time spent watching TV/playing videogames (− 32 and − 25 min·day−1, respectively). In response to an exercise stimulus, previously inactive adults make encouraging modifications in how they use their time, and the patterns of change are similar with morning and evening exercise.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10865-022-00370-x.