  The concept of natural time turned out to be useful in revealing dynamical
features behind complex time series including electrocardiograms, ionic current
fluctuations of membrane channels, seismic electric signals, and seismic event
correlation. However, the origin of this empirical usefulness is yet to be
clarified. Here, it is shown that this time domain is in fact optimal for
enhancing the signals in time-frequency space by employing the Wigner function
and measuring its localization property.
