Interpretation of normalized spectral heart rate variability indices in sleep research: a critical review.

The normalized spectral heart rate variability (HRV) measures low-frequency (LF)nu and high-frequency (HF)nu are frequently used in contemporary sleep research studies to quantify modulation of the sympathetic and parasympathetic branches of the autonomic nervous system. The purpose of this tutorial and methodologic critique is to concisely demonstrate the structural algebraic redundancy inherent in the normalized spectral HRV measures with respect to each other, and also with respect to the well-known HRV index of sympathovagal balance, LF:HF ratio. The statistical problems and interpretational paradoxes related to the mathematical definitions of LFnu and HFnu are briefly outlined. Examples of use of normalized spectral HRV measures in recent articles from the sleep-relevant research literature are critically reviewed. LFnu, HFnu, and LF:HF ratio should be considered equivalent carriers of information about sympathovagal balance.