Yoga Nidra relaxation increases heart rate variability and is unaffected by a prior bout of Hatha yoga

The measurement of heart rate variability (HRV) is often applied as an index of autonomic nervous system (ANS) balance and, therefore, myocardial stability. Previous studies have suggested that relaxation or mind-body exercise can influence ANS balance positively as measured by HRV but may act via different mechanisms. No studies, to the authors' knowledge, have examined the acute response in HRV to interventions combining relaxation and mind-body exercise. The objective of this study was to compare the acute HRV responses to Yoga Nidra relaxation alone versus Yoga Nidra relaxation preceded by Hatha yoga.This was a randomized counter-balanced trial.The trial was conducted in a university exercise physiology laboratory.Subjects included 20 women and men (29.15±6.98 years of age, with a range of 18-47 years).Participants completed a yoga plus relaxation (YR) session and a relaxation only (R) session.The YR condition produced significant changes from baseline in heart rate (HR; beats per minute [bpm], p0.001) and indices of HRV: R-R (ms, p0.001), pNN50 (%, p=0.009), low frequency (LF; %, p=0.008) and high frequency (HF; %, p=0.035). The R condition produced significant changes from baseline in heart rate (bpm, p0.001) as well as indices of HRV: R-R (ms, p0.001), HF (ms(2), p=0.004), LF (%, p=0.005), HF (%, p=0.008) and LF:HF ratio (%, p=0.008). There were no significant differences between conditions at baseline nor for the changes from baseline for any of the variables.These changes demonstrate a favorable shift in autonomic balance to the parasympathetic branch of the ANS for both conditions, and that Yoga Nidra relaxation produces favorable changes in measures of HRV whether alone or preceded by a bout of Hatha yoga.