tokens: Although the QT duration was higher ( p = 0 . 0001 ) at night , the beat - to - beat variability of this interval was lower : in the time domain ( decreased standard deviation , p = 0 . 0005 ) , in the frequency domain ( decreased low - frequency power of the spectra , p = 0 . 004 ) , and the chaotic domain ( tighter clustering of the points in the Poincare plots ) .