Heart rate variability during wakefulness reflects sleep apnea indicators but remains context-dependent

Sleep-disordered breathing (SDB), particularly obstructive sleep apnea (OSA), is a major public health issue linked to cardiovascular morbidity and mortality. While polysomnography (PSG) remains the diagnostic gold standard, its complexity and cost limit widespread use. Heart rate variability (HRV) has emerged as a promising non-invasive alternative for assessing autonomic dysregulation associated with SDB, even when measured outside sleep periods. However, its reliability in extreme settings remains unclear. This study investigates whether time-domain HRV parameters measured during wakefulness reflect nocturnal SDB severity across two populations: 11 healthy men exposed to prolonged high-altitude hypoxia during a one-year stay at the Antarctic Concordia station (equivalent to ~3,800 m) and 35 clinically suspected OSA men. HRV metrics were compared against the apnea-hypopnea index (AHI) and pulse oximetry-derived respiratory indices. In the clinical group, HRV showed significant associations with OSA severity, including a negative correlation between root mean square of successive differences (RMSSD) and AHI (r = -0.524, p = 0.001) and a positive correlation between RMSSD and mean nocturnal oxygen saturation (SpO 2 ) (r = 0.703, p < 0.001). In the high-altitude group, weaker but significant longitudinal associations were observed only in nights without PSG recordings, including correlations between RMSSD and SpO 2 (r = 0.339, p = 0.016), and between deceleration capacity and SpO 2 (r = -0.200, p = 0.009). While HRV may not serve as a definitive diagnostic marker, it could function as an early indicator of physiological stress and potential SDB, particularly in resource-limited or controlled environments. These findings underscore the need for context-specific validation of HRV-based screening tools prior to clinical implementation.