Clinicians have traditionally described the normal activity of the heart as ``regular sinus rhythm.'' However, contrary to subjective impression and clinical assumption, cardiac interbeat intervals normally fluctuate in a complex, apparently erratic manner, even in individuals at rest (Fig. 1a) [1, 19]. This highly irregular behavior defies conventional analyses that require ``well-behaved'' (stationary) data sets. Fractal analysis techniques developed above are good candidates for studying this type of time series where fluctuations on multiple time scales appear to occur.