Heart Rate Variance and Breathing Coherence
Research over the last couple of decades has shown heart rate variability (the change in heart rate over time) to be an indicator of certain body states. For instance, when you get into a coherent state where your heart rate increases on in breaths and decreases on out breaths, you are able to feel and be more healthy, calm, and in the zone, etc.
Scientists are studying the effects of this coherent state on a number of different conditions including: asthma, COPD, IBS, cyclic vomiting, recurrent abdominal pain, fibromyalgia, cardiac rehabilitation, hypertension, chronic muscle pain, pregnancy induced hypertension, depression, anxiety, PTSD, insomnia, and performance.
This article goes into much greater detail.
I've written a small application to start playing with heart rate data to help people get into a coherent state. I believe it's possible to integrate the heart rate data into a virtual reality environment and gamify an experience that leads people in this coherent state.