The purpose of this study was to evaluate the feasibility and acceptability of randomizing adults with overweight and obesity (BMI 25 to 40 kg/m2) to morning (06:00 to 10:00) or evening (15:00 to 19:00) aerobic exercise. Participants completed four exercise sessions per week in the morning (AM, n = 18) or evening (PM, n = 15). The exercise program was 15 weeks and progressed from 70 to 80% heart rate maximum and 750 to 2000 kcal/week. Bodyweight, body composition, total daily energy expenditure (TDEE), energy intake (EI), sleep, sedentary behavior (SB), non-exercise physical activity (NEPA), and maximal aerobic capacity were assessed at baseline and week 15. Study retention was 94% and adherence to the supervised exercise program was >= 90% in both groups. Weight change was -0.9 +/- 2.8 kg and -1.4 +/- 2.3 kg in AM and PM, respectively. AM and PM increased TDEE (AM: 222 +/- 399 kcal/day, PM: 90 +/- 150 kcal/day). EI increased in AM (99 +/- 198 kcal/day) and decreased in PM (-21 +/- 156 kcal/day) across the intervention. It is feasible to randomize adults with overweight and obesity to morning or evening aerobic exercise with high levels of adherence. Future trials are needed to understand how the timing of exercise affects energy balance and body weight regulation.

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