PURPOSE: Recommendations state that to acquire the greatest benefit from heat acclimation the clock-time of heat acclimation sessions should match the clock-time of expected exercise-heat stress. It remains unknown if adaptations by post-exercise hot water immersion (HWI) demonstrate time of day dependent adaptations. Thus, we examined whether adaptations following post-exercise 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 (0945-h: AM) and afternoon (1445-h: PM; 40 min; 65% V̇O2max treadmill run) before (PRE) and after (POST) heat acclimation. The 6-day heat acclimation intervention involved a daily, 40 min treadmill-run (65% V̇O2max) in temperate conditions followed by ≤ 40 min HWI (40°C; 0630-1100-h).

RESULTS: Adaptations by 6-day post-exercise HWI in the morning were similar in the morning and afternoon. Reductions in resting rectal temperature (Tre; AM; -0.34 ± 0.24°C, PM; -0.27 ± 0.23°C; P = 0.002), Tre at sweating onset (AM; -0.34 ± 0.24°C, PM; -0.31 ± 0.25°C; P = 0.001), and end-exercise Tre (AM; -0.47 ± 0.33°C, PM; -0.43 ± 0.29°C; P = 0.001), heart rate (AM; -14 ± 7 beats∙min-1, PM; -13 ± 6 beats∙min-1; P < 0.01), rating of perceived exertion (P = 0.01), and thermal sensation (P = 0.005) were not different in the morning compared to the afternoon.

CONCLUSION: Morning heat acclimation by post-exercise hot water immersion induced adaptions at rest and during exercise-heat stress in the morning and mid-afternoon.