Heart rate variability (HRV)

Heart rate variability is the amount of variation in the time interval between consecutive heartbeats, measured in milliseconds. Rather than a fixed rhythm like a metronome, a healthy heart speeds up and slows down slightly from beat to beat, and how much it varies reflects the balance between the sympathetic ("fight or flight") and parasympathetic ("rest and digest") branches of the autonomic nervous system.

HRV is typically captured from an ECG or, in consumer wearables, an optical pulse sensor, and often summarized with a statistic such as RMSSD, the root mean square of successive differences between beats. This differs from resting heart rate itself: two people, or the same person on two different days, can have the same average heart rate but very different variability, and higher HRV is generally associated with better recovery and lower stress, while a sustained drop can signal fatigue, illness, or overtraining.

HRV is widely used in quantified self tracking and by athletes and coaches to guide load management decisions, adjusting training intensity based on how recovered the body appears to be. The main pitfalls are treating a single day's reading as meaningful, since HRV is naturally noisy and better interpreted as a trend, and comparing HRV values across different devices or measurement methods, which are not always consistent with one another.