Purpose: This study investigated the effect of time of day on endurance exercise capacity in a warm environment.
Methods: Nine males cycled to exhaustion at 65% V˙O2peak in an ambient temperature of 35°C (60% relative humidity) at 0645 h (AM) and 1845 h (PM). Rectal temperature (Tc), skin temperature (Tsk), and heart rate (HR) were recorded and blood and expired air samples collected at rest every 5 min during exercise and during recovery.
Results: Time to exhaustion was longer in the AM trial (45.8 ± 10.7 min) than in the PM trial (40.5 ± 9.0 min; P = 0.009). Resting Tc was lower in the AM trial and remained lower for the first 25 min of exercise (P < 0.001). Tc was not different between trials at the point of exhaustion (AM = 38.7 ± 0.9°C, PM = 38.8 ± 0.6°C; P = 0.847). Tsk followed a similar pattern, being lower at rest (P = 0.003) and during the initial stages of exercise (P < 0.05) in the AM trial but not different at exhaustion (P = 0.896). The rate of rise of Tc tended to be greater in the AM trial (P = 0.052), and the rate of rise of Tsk (P = 0.032) and of body heat content (P = 0.009) was also greater in the AM trial. HR was initially higher in the PM trial, but there was no difference at exhaustion. There were no differences between trials in blood glucose concentration or plasma volume change.
Conclusion: The results of this study suggest that endurance exercise capacity in the heat was significantly greater in the morning than the evening, possibly due to a lower initial Tc.