The Healthy Chaos Your heart is not a metronome. If it were, you'd be in serious trouble. The healthy heart speeds up and slows down constantly, responding to breath, to thought, to emotion, to movement, to a thousand signals from your environment and inner state. This variation between beats, measured in milliseconds, is called heart rate variability, and it may be the single most important biomarker you've never heard of. High variability indicates a nervous system that's f