Heart Rate Variability and Sleep Architecture Response to Repeated Bouts of Heat exposure

Purpose: Heat acclimation (HA) is commonly used to improve thermoregulation and reduce heat illness risk in military personnel and athletes. While HA enhances thermoregulation, cardiovascular efficiency, and exercise capacity it may impair recovery when combined with repeated heat exposure accompanied with increased training load. This study assessed recovery patterns using heart rate variability (HRV) and sleep efficiency (SE) measured via a wrist-worn wearable device during a 14-day HA protocol. We hypothesized that repeated heat exposure disrupts recovery by altering sympathetic activation, SE, and sleep architecture. Methods: 51 participants completed a 14-day heat acclimation protocol in an environmental chamber (40ºC, 40% RH), with two heat tolerance tests before (preHA) and after (postHA) 5 days of HA sessions (60-minute treadmill walking/running at T rec >38.5°C). Participants wore a device continuously, and HRV, along with sleep parameters (SE, REM, light, deep sleep), were measured via PPG and actigraphy. Timepoints with fewer than 20 data points were excluded, and a reviewed dataset was used for statistical analysis. Bonferroni-corrected dependent t-tests and one-way ANOVA were applied to recovery markers and sleep parameters. Results: Acclimation status was confirmed by significant changes in average heart rate (aveHR) and rectal temperature (T rec ) from preHA to postHA (p<0.01). Night-time HRV showed no significant changes across timepoints (p>0.05). SE differed significantly between certain days before and after HA (p<0.05). REM sleep showed no significant changes (p>0.05). Light sleep was significantly reduced post-acclimation, with POSTDAY9 and POSTDAY10 lower than PREB2 (p=0.017; p=0.004, respectively). Deep sleep (hours) also significantly differed at various timepoints during HA, with notable changes observed between PREB2, POSTB2, PREHA1, and POSTHA1 (p<0.05). A one-way ANOVA showed no significant changes from baseline for deep and light sleep stages, but a significant time effect for REM sleep (p=0.048). Correlational analysis between rectal temperature at 120 minutes (T rec @120) during HTT2 and HRV and SE showed a weak negative correlation preHA (r=-0.34;r=-0.18), which weakened further postHA (r=-0.17;r =-0.05). T rec @120 and total sleep time (hours) shifted from a weak negative to a weak positive correlation (r=-0.25;r=0.13), while T rec @120 and LIGHT shifted from a weak negative to almost no correlation (r=-0.32;r=0.002), reflecting a negligible relationship as HA progressed. Conclusion: These findings suggest that heat acclimation may not significantly affect sympathetic activation (HRV) but may alter sleep efficiency and architecture, impacting recovery dynamics. The marginal change in the correlation between HA status and recovery values suggest a possible avenue for future research on recovery shifts during heat acclimation. Funding Source: DoD BA200299, DoD BA220010 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.