BACKGROUND: The time of day that people exercise could have an influence on the efficacy of exercise for weight loss, via differences in adherence and/or physiological adaptations. However, there is currently no evidence to support an optimal time of day for exercise to maximise efficacy. PURPOSE(S): To examine the feasibility and acceptability of prescribed morning and evening exercise. METHOD(S): Twenty inactive, overweight adults aged 18 to 60 years were recruited for a 12-week intervention and randomized to one of three groups using a 2:2:1 random allocation ratio: (i) morning exercise (AM; n = 9); (ii) evening exercise (PM; n = 7); or (iii) waitlist control (CON; n = 4). Exercise groups were prescribed self-paced walking or running on a treadmill to achieve a weekly total of 250 min. Feasibility and acceptability data were collected, and physiological and behavioural outcomes associated with energy balance were measured at baseline, mid- and post-intervention. RESULT(S): Attrition was low (n = 2 dropped out), with high measurement completion rates (> 80%). The intervention groups had high adherence rates to exercise sessions (94% and 87% for the AM and PM groups, respectively). No adverse events resulting from the intervention were reported. Both intervention groups displayed improvements to their cardiometabolic risk profile; cardiorespiratory fitness improved by 5.2 +/- 4.7, and 4.6 +/- 4.5 mL/kg/min and body fat percentage reduced by 1.2 +/- 1.4, and -0.6 +/- 1.2% for AM and PM groups, respectively. CONCLUSION(S): This feasibility study provides evidence that morning and evening exercise interventions are feasible, and also provides justification for a large-scale randomized controlled trial. TRIAL REGISTRATION: This trial is registered with the Australian New Zealand Clinical Trials Registry (ACTRN12616000457448p, 7/4/2016).

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