Heat reactivity and nocturnal vagal recovery in volleyball: a multi-centre prospective observational cohort with internal replication

Background & Objective Volleyball is played in thermally divergent environments (e.g., humid indoor arenas vs. radiant outdoor courts). However, the autonomic cost of accumulated heat stress is not uniform across athletes. This study aimed to determine whether a field-derived heart-rate heat-reactivity phenotype modifies the within-athlete association between cumulative thermal load and nocturnal vagal recovery (sleep-period RMSSD), and whether this effect replicates across independent teams and contrasting climates. Methods We conducted a multi-centre prospective observational cohort study involving 78 hard-court volleyball athletes (Discovery n=26; 3-team Validation pool n=52). Exposure: PSI-HR, a heart-rate-anchored cumulative index of cardiovascular regulatory cost (bpm·min). Phenotype: The within-athlete slope of heart rate on WBGT during intensity-matched sessions, dichotomised at the median into heat-sensitive and heat-tolerant groups. Outcome: PPG-derived sleep-period RMSSD (Garmin Epix 2). Analysis: Three-level linear mixed models tested the interaction, supported by 9 prespecified sensitivity analyses. Key Findings A significant interaction was observed (P = 0.018). Heat-sensitive athletes showed a significantly steeper decline in sleep-period RMSSD per 1,000 bpm·min of thermal load (–4.42 ms) compared to heat-tolerant athletes (–0.61 ms). The between-stratum difference was –3.81 ms. Notably, this effect replicated perfectly across the discovery and validation pools, with negligible between-team heterogeneity (I² = 0%). All sensitivity models (including continuous phenotype and signal-quality restrictions) were concordant. Significance & Impact The effect size is ~2.9× the 3% smallest worthwhile change used in athlete monitoring. This field-deployable phenotype can help practitioners identify athletes at greater risk of vagal suppression following heat exposure. However, due to wrist-PPG measurement error, this tool is currently suitable for group-level monitoring only, not individual heat-safety classification. Limitations Exposure was observational, and no laboratory core-temperature measures were available. The phenotype's construct validity as a heat-tolerance metric requires further laboratory validation.