To evaluate the high-frequency component (HF) of the heart rhythm, a recording of about 1 min is needed, whereas for the analysis of the low-frequency component (LF), 2 minutes of recording are required. For an objective assessment of the very low-frequency component of HRV (VLF), the recording time must be at least 5 minutes. Therefore, for the standardization of studies of HRV with short recordings, a preferred recording duration of 5 minutes was chosen.