Table 1 Explanation of the examined spectral bands

VLF

Very low frequency or thermoregulation band:

Power spectral density (PSD) in the frequency range up to 0.04 Hz (≤ 0.04 Hz), period duration from 25 s to 5:30 min, here the sympathetic nervous system is involved

Thermoregulation related fluctuations, hormonal rhythms

LF

Low Frequency Power or Baroreflector band:

Power spectral density (PSD) in the frequency range 0.04–0.15 Hz, period duration from 7 to 25 s; here both the sympathetic and the parasympathetic nervous system are involved

Blood pressure related fluctuations

HF

High Frequency Power or Respiratory sinus arrhythmia, respiratory band:

Power spectral density (PSD) in the frequency range 0.15–0.40 Hz, period duration from 2.5 bis 7 s; shows only the parasympathetic function

Breathing related fluctuations