Hill, D. W. (1996). Effect of time of day on aerobic power in exhaustive high-intensity exercise. Journal of Sports Medicine and Physical fitness, 36, 155-160.

College students (M = 12; F = 12) each performed three exhaustive high-intensity constant-power cycle ergometer tests. The first was a learning or acclimation trial. The second and third tests were scheduled at 8:00 AM and 4:00 PM, respectively.

Time to exhaustion was 9% greater, peak VO2 was 7% greater, and the aerobic system responded 6% faster, in the PM than AM.

Implication. The aerobic system responds faster and better in the PM than AM.

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