Heart rate variability (HRV) is a measurement of the variation in time intervals between heartbeats. It is a common indicator of autonomic nervous system activity, with high HRV reflecting a healthy and flexible system, while low HRV is often associated with stress, fatigue, and other health issues. HRV is widely used in research studies and clinical settings to assess and monitor cardiovascular health, stress levels, and overall well-being. It can be measured using various devices and techniques, such as electrocardiography and heart rate monitors.