BioMedCentral, the Open Access Publisher is a searchable site for research articles. A general search with the most up to date articles can be made for various topics on HRV and Yoga research, pilot studies, research protocols, etc. by clicking here:

The following are a selection of those research articles on the improvement of HRV with yoga:

Increased heart rate variability but no effect on blood pressure from 8 weeks of hatha yoga – a pilot study
Marian E Papp, Petra Lindfors, Niklas Storck and Per E Wändell
Published on: 11 February 2013

Results – There was no significant effect of inversion postures on BP. Nine out of 12 participants showed a significant increase in HRV (p < 0.05) at night (2 hours) on pNN50% (12.7 ± 12.5 to 18.2 ± 13.3). There were no significant changes in other HRV measures such as NN50, LF, HF, LF/HF ratio, LF normalized units (n.u.), HF n.u. and RMSSD.