Rising vs. falling phases of core temperature on endurance exercise capacity in the heat

Core temperature (T_c) shows rising (05:00-17:00 h) and falling (17:00-05:00 h) phases. This study examined the time-of-day effects on endurance exercise capacity and heat-loss responses to control T_c in the heat at around the midpoint of the rising and falling phases of T_c.

Ten male participants completed cycling exercise at 70% peak oxygen uptake until exhaustion in the heat (30 °C, 50% relative humidity). Participants commenced exercise in the late morning at 10:00 h (AM) or evening at 21:00 h (PM).

Time to exhaustion was 28 ± 13% (mean ± SD) longer in PM (49.1 ± 16.3 min) than AM (38.7 ± 14.6 min; P < 0.001). T_c before and during exercise were higher in PM than AM (both P < 0.01) in accordance with the diurnal variation of T_c. The rates of rise in T_c, mean skin temperature, thermal sensation and rating of perceived exertion during exercise were slower in PM than AM (all P < 0.05). Dry and evaporative heat losses and skin blood flow during exercise were greater in PM than AM (all P < 0.05). During 30-min post-exercise recovery, the rates of fall in T_c and skin blood flow were faster and thermal sensation was lower in PM than AM (all P < 0.05).

This study indicates that endurance exercise capacity is greater and heat-loss responses to control T_c during and following exercise in the heat are more effective in the late evening than morning. Moreover, perceived fatigue during exercise and thermal perception during and following exercise are lower in the late evening than morning.