Fig. 13.

(a) Decrease of the fractal dimension (FD) of HRV with age in male subjects, $r=-0.56(p<0.001)$, and (b) in female subjects, $r=-0.56(p<0.001)$. The upper and lower lines represent the 90% confidence intervals. There is no significant difference in FD between male and female subjects, but the significant decrease with age persists regardless of whether the measurements were made during the day or night. A more detailed analysis using age intervals of 10 years showed a stabilization in the age decline of the FD at the age of 40 or more. The FD has correlation with other nonlinear indices such as ApEn, DFA exponents, and Lyapunov exponent. The figure is taken from [129].