Passive heat acclimation does not modulate processing speed and executive functions during cognitive tasks performed at fixed levels of thermal strain.

This study evaluated if passive controlled hyperthermia heat acclimation modulates cognitive performance during passive heat stress. Eight healthy adults (25 ± 4 years) underwent 7 consecutive days of hot water immersion (core temperature ≥38.6 °C) and a 7-day time-control period. On days 1 and 7 of heat acclimation, participants performed a digital Stroop test at baseline, when core temperature reached 38.6 °C, and after 60 minutes at a core temperature ≥38.6 °C to evaluate reaction time during tasks targeting processing speed (reading and counting) and executive functions (inhibition and switching). On days 1 and 7 of the time-control intervention, participants performed the Stroop test with equivalent amounts of time separating each task as for heat acclimation. During day 1 of heat acclimation, reaction time was quicker during the reading (-44 ms [-71 to -17],  < 0.01) and counting (-39 ms [-76 to -2],  = 0.04) tasks when the rectal temperature reached 38.6 °C, but after a further 60 minutes of heat exposure, reaction time only remained quicker during the reading task (-56 ms [-83 to -29],  < 0.01). Changes in reaction time during heat exposure were unaffected by subsequent heat acclimation (interaction, all  ≥ 0.09). In conclusion, 7 days of heat acclimation does not modulate processing speed and executive functions during passive heat exposure. Whether heat acclimation improves cognitive performance during heat exposure remains unclear. We tested the hypothesis that heat acclimation modulates reaction time during cognitive tasks performed at matched levels of thermal strain. Despite the classical signs of heat acclimation, reaction time during heat exposure is unaffected by heat acclimation.