Most short- and medium-term (up to one hour) measurements of heart rate variability are themselves highly variable over time. This intra-subject variability limits the value of inter-subject comparisons of these measurements, unless periods of comparable physical activity can be matched. With the goal of being able to make such comparisons in the absence of independent activity data, this study investigated methods for finding periods of minimum activity based on the ECG alone. Such periods are well-suited for HRV comparisons, since minimum activity levels are usually comparable even when maximum or average activity levels are not. Independent of these considerations, inter-beat intervals (hence HRV measurements derived from them) are usually measured most accurately during the noise-free ECG segments that occur most often during periods of minimum activity.