This is a Plos One, so the whole article is available online.  They make that case that positive affect is correlated with high HRV. I Been doing a lot of reading today, and the autonomic nervous system is involved in everything. 

Affective Instability in Daily Life Is Predicted by Resting Heart Rate Variability

Peter Koval,#1 Barbara Ogrinz,#1 Peter Kuppens,1 Omer Van den Bergh,1 Francis Tuerlinckx,1 and Stefan Sütterlin1,2,3,*

Previous research has shown that being affectively unstable is an indicator of several forms of psychological maladjustment. However, little is known about the mechanisms underlying affective instability. Our research aims to examine the possibility that being prone to extreme fluctuations in one’s feelings is related to maladaptive emotion regulation. We investigated this hypothesis by relating affective instability, assessed in daily life using the experience sampling method, to self-reported emotion regulation strategies and to parasympathetically mediated heart rate variability (HRV), a physiological indicator of emotion regulation capacity. Results showed that HRV was negatively related to instability of positive affect (as measured by mean square successive differences), indicating that individuals with lower parasympathetic tone are emotionally less stable, particularly for positive affect.