Within-Subject Association Between Daily Energy Availability and Next-Night Nocturnal Heart Rate Variability in Active Adults

Lucie Saclova1, Andrea Nemcova2, Tobiáš Goldschmidt3, Lukas Smital2, Daniela Chlibkova4, Martin Kralik1, Jaromír Hubálek1
1Brno University of Technology, 2Brno University of Technology, Faculty of Electrical Engineering and Communication, Department of Biomedical Engineering, 3Department of Biomedical Engineering, Faculty of Electrical Engineering and Communication, Brno University of Technology, 4Centre of Sport Activities, Brno University of Technology

Abstract

Introduction: The within-subject association between day-to-day energy availability (EA) and next-night nocturnal heart rate variability (HRV) remains insufficiently characterized. Low EA (LEA, <30 kcal·kg⁻¹ fat free mass·day⁻¹) is the underlying mechanism of Relative Energy Deficiency in Sport (RED-S), yet its acute autonomic correlates are not well resolved at the day-to-day level. We quantified the within-person association between daily EA and next-night nocturnal HRV in 22 physically active individuals (BUT CGM dataset) who experienced both LEA and adequate EA ( ≥45 kcal·kg⁻¹ fat free mass·day⁻¹) days during 8–10 days of free-living monitoring (123 person-day observations; 71 LEA, 52 adequate EA). Methods: Daily EA was computed as (energy intake − exercise energy expenditure) / fat-free mass. Exercise energy expenditure was estimated using a cardiopulmonary exercise testing (CPET) calibrated zone-based model with individual ventilatory thresholds and respiratory exchange ratio–derived caloric equivalents. Nocturnal HRV was extracted from single-lead ECG during sleep windows, computed on 5-minute segments using time-domain and Welch frequency-domain methods, and averaged across the night; each night was paired with the preceding day's EA. Results: Within-person adequate EA − Low EA contrasts were estimated using linear mixed-effects models with random intercepts for participants. Eight of ten HRV parameters were significantly elevated on adequate EA versus LEA days: pNN50, HF power, RMSSD, standard deviation of heart rate (SDHR), SDNN, LF power, lnRMSSD, and RR intervals. Heart rate and LF/HF ratio did not differ.
Conclusion: These findings characterize the within-subject association between day-to-day EA and nocturnal HRV under free-living conditions, indicating that vagal-mediated and overall variability indices are systematically reduced on LEA days. Nocturnal HRV may thus serve as a sensitive, non-invasive marker of short-term physiological status in physically active individuals; however, observed changes in HRV may reflect multiple influences, with LEA representing one of the potential contributing factors.