The same HRV. A completely different state. HRV is often used today as a marker for stress, recovery, or performance. However, HRV alone initially describes only temporal variations between heartbeats — not the underlying cause of these variations. This is the central challenge of interpretation. Stress, fatigue, breathing, physical activity, training, illness, medication, or alcohol each affect physiology in different ways. Yet they can produce similar or even identical HRV values. Therefore, the same HRV does not automatically represent the same physiological condition. Without additional context, HRV often remains interpretation rather than a clear physiological statement. Only physiological calibration allows differentiation between regulation, adaptation, stress, compensation, or emerging dysregulation. Calibration using second-order blood pressure fluctuations provides the missing physiological context. This transforms HRV from a purely temporal signal into a significantly more meaningful physiological interpretation of human adaptability. HRV shows change. Physiology explains the cause. kurz The same HRV can have completely different causes. Stress, fatigue, training, illness, or breathing each change physiology differently. HRV alone does not know the cause. Only calibration with second-order blood pressure fluctuations provides the physiological context.