Interpreting Heart Rate Variability in Sleep: Why, When, and How?

Sleep and the autonomic nervous system (ANS) are biologically and clinically associated. Neuronal pathways located in the brain stem and basal forebrain responsible for the wake–sleep transition are connected with areas of the central nervous system regulating ANS activity. Indeed, at the forebrain diencephalic level, sleep itself may be considered to be one of the most highly integrated autonomic functions in which behavioral and homeostatic integration occurs. In turn, this integration is bidirectionally interconnected with the other hierarchical levels. Thus, the peripheral autonomic motor activity (traditionally separated into sympathetic and parasympathetic components) cannot be considered truly autonomous, but rather an element of somatic and visceral motor regulation which occurs in various behaviors. Attention to the powerful capacity of the continuous inflow of animal spirits into the heart may help clarify the difference between wakefulness and death and how the system may be driven to change from its natural constitution. In this chapter, we will briefly summarize some of the physiopathological background that might help address the questions that are suggested by the vast amount of information continuously produced on the link between sleep and autonomic (dys)regulation. We do not delve so much, however, into the continuously evolving technical aspects (how and when) but wish to help interested readers to address the critical question that hovers at the heart of all experimental work and regards the choice of the appropriate technique of investigation: why should we use heart rate variability (HRV)?