Sex Differences in Sleep and Recovery Response with Multiple Heat Exposures for Highly Trained Endurance Runners

Objective: The purpose of this study was to investigate the effect of multiple heat exposures on sleep and recovery between trained male and female athletes. Hypothesis: We hypothesized that there will be no differences in sleep and recovery responses between males and females. METHOD: Nine high-level endurance runners (3 males [M]; age: 22±4 years, maximal oxygen uptake [VO 2max ]: 75.6±11.9 ml·kg -1 ·min -1 , 6 females [F]; age: 23±5 years, VO 2max : 57.9±4.6 ml·kg -1 ·min -1 ) completed 7 days of 60 min running exercise at 50% VO 2max in the heat (HEAT) (40 °C, 35 %) and temperate (TEMP) (22°C, 30%) conditions in a random order. Heart rate (HR) and rectal temperature (T rec ) were collected during exercise. Additionally, participants wore a validated smart ring at night throughout the study period to monitor their sleep and recovery metrics. Linear mixed effects models with Tukey post hoc analysis were performed to examine sex differences in response to exercise in heat and temperate conditions. Participants were categorized as the random effect and the environmental condition and sex were used as the fixed effects. Results: There were no sex differences in maximum HR (HEAT_F: 174±10 bpm, TEMP_F: 140±12 bpm, HEAT_M: 177±9 bpm, TEMP_M: 146±11 bpm) and T rec (HEAT_F: 39.2±0.6 °C, TEMP_F: 38.1±0.3°C, HEAT_M: 39.3±0.3 °C, TEMP_M: 38.2±0.4 bpm) within the same environmental conditions (p>0.05), however, they were higher during exercise in the heat for both sexes (p<0.001). Average resting HR during sleep was higher following heat exposures in females (HEAT: 54±6 bpm, TEMP: 51±7 bpm, p<0.001), however, it was lower in males (HEAT: 62±3 bpm, TEMP: 67±9 bpm, p=0.0016). Average heart rate variability was lower following heat exposures compared to temperate conditions in females (HEAT: 86±38 ms, TEMP: 99±38 ms, p<0.001), but there was no difference in males (HEAT: 32±8 ms, TEMP: 22±6 ms, p=0.6418). Similarly, respiratory rate was higher following heat exposure in females (HEAT: 17±2 bpm, TEMP: 16±2 bpm, p=0.0461), but no difference in males (HEAT: 14±1 bpm, TEMP: 15±1 bpm, p=0.4353). Sleep duration (HEAT_F: 6.7±1.6 hr, TEMP_F: 6.9±1.5 hr, HEAT_M: 7.3±1.8 hr, TEMP_M: 5.7±1.0 hr), rapid eye movement (REM) sleep percentage (HEAT_F: 23±5 %, TEMP_F: 24±6 %, HEAT_M: 22±6 %, TEMP_M: 24±8 %), REM sleep duration (HEAT_F: 1.6±0.5 hr, TEMP_F: 1.7±0.5 hr, HEAT_M: 1.6±0.6 hr, TEMP_M: 1.4±0.5 hr), slow wave sleep (SWS) percentage (HEAT_F: 19±5 %, TEMP_F: 21±5 %, HEAT_M: 23±5 %, TEMP_M: 26±9 %), SWS duration (HEAT_F: 1.2±0.4 hr, TEMP_F: 1.4±0.3 hr, HEAT_M: 1.7±0.3 hr, TEMP_M: 1.4±0.5 hr), and sleep efficiency (HEAT_F: 90±5 %, TEMP_F: 89±5 %, HEAT_M: 90±3 %, TEMP_M: 86±14 %) were not different between males and females or following exercise in the heat compared to temperate condition (p>0.05). Conclusion: Recovery and readiness responses to exercise in the heat compared to the temperate condition were different between male and female athletes while there was no difference in sleep. These results suggest that sex-specific approaches might be required to promote recovery and readiness to heat exposures. This study was funded by American College of Sports Medicine World Athletics Federation Research Grant. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.