The time between individual beats of the heart, often measured using ECG (see “ECG/Electrocardiography”). The inter-beat interval (IBI) is most often measured by the time interval between R-Waves [a specific deflection in the ECG trace used to calculate heart rate (see “Heart Rate”)]. Heart Rate Variability (HRV) is a derived analysis of the variability of these intervals which looks at how heart rate changes over a period of time.