When we explain that heart rate variability (HRV) is the variation in timing between heartbeats, and that the higher the HRV (the greater the variation), the more resilient and healthy you are likely to be—well, that’s confusing. Aren’t irregular heartbeats a sign of atrial fibrillation?

In atrial fibrillation, the heart beats too fast, or too slow, or irregularly. In any of these cases, however, the heart is beating inefficiently, causing poor oxygenation and other problems. The key is that the heart is beating abnormally, caused by a problem in the heart’s electrical system.

HRV is created by the normal tug-of-war between the body’s parasympathetic and sympathetic nervous systems. The differences in timing from one beat to the next are in terms of microseconds. High HRV signifies low stress and robustness because it shows the body is highly responsive to minute changes in its internal and external environments. Low HRV is a sign that the body is tired, ill, or stressed, and therefore no longer as quick to respond.