Morning–evening differences in response to exhaustive severe-intensity exercise

Abstract

The aim was to investigate the effect of time of day on 4 variables that are related to sport performance. Twenty healthy young men (mean ± SD: 22 ± 3 years, 1.78 ± 0.08 m, 72.0 ± 7.0 kg) performed exhaustive severe-intensity cycle ergometer tests at 278 ± 35 W (3.8 ± 0.4 W·kg–1) in the morning (between 0630 h and 0930 h) and in the evening (between 1700 h and 2000 h). Despite that gross efficiency was lower in the evening (estimated oxygen demand was 6% higher, P < 0.05), time to exhaustion was 20% greater (P < 0.01) in the evening (329 ± 35 s) than in the morning (275 ± 29 s). Performance in the evening was associated with a 4% higher (P < 0.01) maximal oxygen uptake (54 ± 7 mL·kg–1·min–1 vs. 52 ± 6 mL·kg–1·min–1, for the evening and the morning, respectively) and a 7% higher (P < 0.01) anaerobic capacity (as reflected by maximal accumulated oxygen deficit: 75 ± 9 mL·kg–1 vs. 70 ± 7 mL·kg–1, for the evening and the morning, respectively). In addition, oxygen uptake kinetics was faster in the evening, which resulted in slower utilization of the anaerobic reserves. It is concluded that modest morning–evening differences in maximal oxygen uptake, anaerobic capacity, and oxygen uptake kinetics conflate to produce a markedly longer performance in the evening than in the morning. Time of day must be considered for exercise testing and perhaps for exercise training.