  We present the first systematic evidence for the origins of 1/f-type temporal
scaling in human heart rate. The heart rate is regulated by the activity of two
branches of the autonomic nervous system: the parasympathetic (PNS) and the
sympathetic (SNS) nervous systems. We examine alterations in the scaling
property when the balance between PNS and SNS activity is modified, and find
that the relative PNS suppression by congestive heart failure results in a
substantial increase in the Hurst exponent H towards random walk scaling
$1/f^{2}$ and a similar breakdown is observed with relative SNS suppression by
primary autonomic failure. These results suggest that 1/f scaling in heart rate
requires the intricate balance between the antagonistic activity of PNS and
SNS.
