Heart rate variability code: Does it exist and can we hack it?

Heart rate variability (HRV) has been studied for over 50 years, yet an integrative concept is missing on what HRV's mathematical properties represent physiologically. Here I introduce the notion of HRV code as an attempt to address this challenge systematically. I review the existing evidence from physiological studies in various species to support this concept and propose experiments to help validate and expand this notion further.