Scheduling Sprint Interval Training at a Constant Rather Than Variable Time of Day Does Not Influence the Gains in Endurance Performance

Introduction: The benefits of consistently exercising at the same time of day are currently unknown. The endurance benefits (10km cycle time-trial) of sprint interval training (SIT) were compared when SIT (six sessions of 4-to-7 x 30-second sprints) was completed at a constant or variable time of day.Methods: Twenty-six adults were assigned to a training schedule: Constant- training and pre/post (Post1) time-trials occurred at a fixed time of day; Variable- training never occurred within 6-hours of the time of day of the previous session; post-SIT time-trials (Post1) for the Variable group were completed at the same time of day as pre-SIT time- trials. A second post-SIT time-trial (Post2) occurred ≥6-hours earlier/later than the time of day of (Post1).Results: The coefficient of variation of oral temperature, measured prior to each SIT session, was greater (P=0.02) in the Variable compared with the Constant group. SIT improved time-trial performance (P<0.001) at Post2; the magnitude of improvement was not different between schedules (Constant: Pre 1158±164, Post1 1144±165, & Post2 1124±146 s vs. Variable: Pre 1216±136, Post1 1189±130, & Post2 1165±121 seconds (mean±SD); group-x-training P=0.646).Conclusions: Consistently training at the same time of day does not appear to be a critical consideration when scheduling regular exercise.