Circadian modulation of core temperature and thermoregulatory strain during live-fire compartment exposure in...

Firefighters are repeatedly exposed to extreme radiant and convective heat during live-fire training, yet the potential influence of circadian timing on their thermoregulatory tolerance remains unexplored. This study tested whether time-of-day modulates physiological strain during standardized container fire exposures. Twenty-one professional male firefighters completed two identical 40-min live-fire sessions on the same day: late-morning (09:00 h, heat-gain phase) and late-evening (21:30 h, heat-loss phase). Core temperature (ingestible sensor), heart rate, skin temperature, under-PPE temperature and humidity, body mass, total body water, and ratings of perceived exertion were recorded. Environmental conditions were strictly matched between sessions. Core temperature rose faster and higher in the morning (ΔTcore +1.10 ± 0.25 °C; slope 0.028 °C·min^-1) than in the evening (+0.49 ± 0.21 °C; 0.012 °C·min^-1), despite similar peak values. Post-exposure cooling was slower in the morning (-0.37 vs -0.63 °C·h^-1), with a delayed hypothermic rebound. Morning sessions also elicited higher heart rates, greater perceived exertion, larger body-mass and water losses, and higher sub-garment humidity. These findings demonstrate that circadian phase significantly influences heat storage and recovery, with late-morning exposures imposing greater thermophysiological strain under identical workloads. Incorporating chronobiological principles into firefighter training schedules may reduce heat-related risk and optimize recovery strategies in extreme environments.