My poster project looked at the relative contributions of sympathetic HRV activity (flight/fight) and parasympathetic HRV activity (rest and digest) with ear stimulation (Transcutaneous Vagal Nerve Stimulation TAVNS). I use data from 5 patients successfully treated with acupuncture, two with migraines, 2 with allergies, and one with intention tremor.
I couldn't prove my hypothesis with the data I collected. That means the TAVNS did not improve the vagal tone more than acupuncture alone.
But negative studies are still of value, but not very exciting. (At this point, someone has to suggest we remember all the negative experiments that Edison did in inventing the light bulb, sigh.) Since then, I've seen some studies that suggest the real benefit from TAVNS is the following day.