Heart rate variability in normal and pathological sleep

Sleep is a physiological process involving different biological systems, from molecular to organ level;its integrity is essential for maintaining health and homeostasis in humanbeings. Although in the past sleep has been considered a state of quiet experimental and clinical evidences suggest a note worthy activation of different biological systems during sleep. A key role is played by the autonomic nervous system(ANS), whose modulation regulates cardiovascular functions during sleep on set and different sleep stages. Therefore,an interest on the evaluation of autonomic cardiovascular control in health and disease is growing by means of linear and nonlinear heart rate variability(HRV)analyses. The application of classical tools for ANS analysis, such as HRV during physiological sleep,showed that therapid eye movement(REM) stage is characterized by a like lysympathetic predominance associated with avagal with drawal, while the opposite trend is observed during non-REM sleep. Morerecently, the use of non-linear tools, such as entropy derived indices, have provided new insight on the cardiac autonomic regulation, revealing for instance changes in the cardiovascular complexity during REM sleep supporting the hypothesis of a reduced capability of the cardiovascular system to deal with stress challenges. Interestingly,different HRV tools have been applied to characterize autonomic cardiac control in different pathological conditions, from neurological sleep disorders to sleep disordered breathing(SDB).Insummary, linearandnon-linear analysis of HRV are reliable approaches to assesschanges of autonomiccardiac modulation during sleep both in health and diseases. The use of these tools could provide important information of clinical and prognosticrelevance.