Effect of time of day on adaptive response to a 4-week aerobic exercise program.

The circadian effects of an aerobic training program were studied in 3 groups of men who exercised at different times of day. Twelve healthy sedentary men were assigned to morning (9:00-9:30), afternoon (15:00-15:30) or evening (20:00-20:30) exercise groups. Each group performed a 30-minute 60% VO2max cycle ergometer exercise 4 days per week over a 4-week period. Maximal oxygen uptake (VO2max) was estimated and adaptive responses of heart rate and blood lactate levels to the training program were measured. After 4 weeks, the afternoon group showed a significant increase in estimated VO2max. A significant decrease in heart rate and blood lactate responses occurred in the afternoon and morning groups and the afternoon and evening groups, respectively. These results suggest that aerobic training is most effective in the afternoon.