If you use any type of personal sensor device like a smart watch or a fitness tracker, you are probably already familiar with the concept of heart rate variability or HRV. You might have seen this statistic interpreted in many different ways -- as a measure of cardiovascular fitness, of stress, of mindfulness or resilience in the face of stress, or even of risk for negative cardiovascular events like heart attack and stroke. What is HRV, and what does it mean? HRV is a measure based on the duration between one heartbeat and the next . This is usually measured in milliseconds per beat -- so, for instance, if your heart beats 60 times per minute, then the average interval between two heartbeats is 100 milliseconds (60 seconds / minute x 1 minute / 60 beats = 1 second per beat or 100 ms per beat). The heart has an off-beat as the chambers expand and contract in rhythm, so the technical definition is based on the type of ticker-tape record of heartbeats that's produced by...