Heart Rate Variability (HRV) is the physiological phenomenon of variation in the time interval between consecutive heartbeats, measured in milliseconds. Rather than a perfectly regular heartbeat, the time between beats fluctuates slightly, reflecting the heart's adaptability to various physiological and environmental conditions

. HRV is controlled by the autonomic nervous system (ANS), which has two main branches: the sympathetic nervous system (fight-or-flight response) and the parasympathetic nervous system (rest-and-digest response). These two systems send competing signals to the heart, causing fluctuations in heart rate and thus HRV

. A higher HRV generally indicates a more adaptable and resilient autonomic nervous system, associated with better emotional regulation, fitness, and overall health, while a low HRV can indicate stress, fatigue, or potential health issues

. HRV is measured using the variation in the RR intervals (the time between successive R-peaks on an ECG), with ECG considered the gold standard for accurate measurement. Other methods include photoplethysmography (PPG) and blood pressure monitoring

. In summary, HRV is a key biomarker reflecting the balance and function of the autonomic nervous system, providing insights into stress levels, recovery, emotional regulation, and cardiovascular health