Purpose: Recommendations state that to acquire the greatest benefit from heat-acclimation, the clock time of heat-acclimation sessions should match that of expected exercise-heat stress. It remains unknown if adaptations by postexercise hot-water immersion (HWI) demonstrate time-of-day-dependent adaptations. Thus, the authors examined whether adaptations following postexercise HWI completed in the morning were present during morning and afternoon exercise-heat stress. Methods: Ten males completed an exercise-heat stress test commencing in the morning (9:45 AM) and afternoon (2:45 PM; 40 min; 65% of maximal oxygen uptake treadmill run) before and after heat-acclimation. The 6-d heat-acclimation intervention involved a daily 40-min treadmill run (65% of maximal oxygen uptake) in temperate conditions followed by =40-min HWI (40°C; 6:30-11:00 AM). Results: Adaptations by 6-d postexercise HWI in the morning were similar in the morning and afternoon. Reductions in resting rectal temperature (Tre) (AM -0.34°C [0.24°C], PM -0.27°C [0.23°C]; P=.002), Tre at sweating onset (AM -0.34°C [0.24°C], PM -0.31°C [0.25°C]; P=.001), and end-exercise Tre (AM -0.47°C [0.33°C], PM -0.43°C [0.29°C]; P=.001), heart rate (AM -14 [7] beats/min, PM -13 [6] beats/min; P<.01), rating of perceived exertion (P=.01), and thermal sensation (P=.005) were not different in the morning compared with the afternoon. Conclusion: Morning heat acclimation by postexercise HWI induced adaptations at rest and during exercise-heat stress in the morning and midafternoon.
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