A Comparative Analysis of Cardiac Sympathetic and Parasympathetic Activity in Sleep Using Linear and Nonlinear Indices

Sleep is characterized by notable decreases in physiological functions such as heart rate and blood pressure, which are also mediated by adaptations in cardiac sympathetic (SA) and parasympathetic activity (PA). This study aimed to investigate the differences in such SA and PA between wakefulness and sleep using both linear and nonlinear approaches. Employing a Laguerre expansion of the Wiener-Volterra autoregressive kernels of heart rate variability (HRV) series, we derived the sympathetic activity index (SAI) and parasympathetic activity index (PAI), along with their ratio. In the frequency domain, we derived the power in the low-frequency (LF) and high-frequency (HF) bands, and also calculated their ratio. On their time-resolved estimation, we also analyzed nonlinear features, including short-scaling ex-ponent α1 through detrended fluctuation analysis. Data were gathered in 18 healthy subjects undergoing a full-night sleep recording. Results show significant differences in SAI and PAI during wakefulness and sleep states, with higher SAI variability observed during wakefulness, and increased PAI during sleep. Notably, nonlinear analysis revealed a higher sympathetic α2 during sleep, suggesting increased complexity in sympathetic activity. These findings underscore the importance of parasympathetic activity during a sleep state, also associated with greater short-scale sympathetic complexity. Further research is needed to explore sleep stage-specific changes and pathological observations.Clinical relevance: Characterizing sleep-specific sympathetic and parasympathetic dynamics is crucial in clinical investigations. The application of nonlinear indices may open new perspectives for understanding autonomic regulation during sleep.