| Summary: | 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: To examine the feasibility and acceptability of prescribed morning and evening exercise. Methods: Twenty inactive, overweight adults aged 18–60 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. Results: 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−1.min−1 and body fat percentage reduced by 1.2 ± 1.4, and −0.6 ± 1.2% for AM and PM groups, respectively. Conclusion: 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). Keywords: Exercise, Time of day, Feasibility, Randomized controlled trial, Energy balance
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