Heart Rate Variability
Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.
HRV can be measured to represent the functionality vagal tone, which cannot be directly measured. Vagal tone is an internal biological process referring to the activity of the vagus nerve (the tenth cranial nerve), which originates in the medulla oblongata of the brainstem. The vagus nerve serves as the key component of the parasympathetic branch of the autonomic nervous system, which homeostatically regulates the resting state of the majority of the body's internal organ systems that operate on a largely subconscious level, such as the heart, lungs, eyes, glands and digestive tract. Due to the regulatory nature of the parasympathetic nervous system (PSNS), vagal activity is continuous, chronic, and passive ("tone" in this usage is analogous to "tension",[2] reflecting a sense of continuous baseline activity, much like muscle tone relies on nervous baseline activity).