Heart Rate Variability (HRV) is a proxy metric for overall readiness. A high HRV appears to be a precursor to flow - a state of consciousness characterized by elevated awareness and performance.
The rate at which you can sustainably train your cognitive and physical performance depends on applying a sweet-spot of stress to your system. Too little and your abilities stagnate. Too much and you risk overtraining. To better understand how your physiology responds to training, it’s useful to look at your autonomic nervous system (ANS).
A function of the ANS is to control the inner processes of which humans have no conscious control, such as the heartbeat, digestion, and unconscious breathing. It’s responsible for teetering you between fight-or-flight stress response (sympathetic) and resting mode (parasympathetic). A healthy ANS rapidly can prepare the body for sudden bursts of performance and should have no trouble calming it down. The heart rate plays a key role in these processes, it provides a proxy metric for ANS readiness.
HRV is a measure of the time between heartbeats, which varies significantly based on the present fitness of your nervous system. A low observed beat-to-beat variance suggests that the heart isn’t prepared to respond to sudden bursts in expected activity. A trending reduction in HRV may signal propensity for overtraining and therefore risk of contracting disease or injury.
On the other hand, a high resting HRV has been related to better emotional and cognitive self-regulation. It’s related to higher performance in memory and attention, and a better response to stress.
You may already have a subjective feel for how you are feeling on a day-to-day basis. However, metrics such as HRV provide a perspective on subtle patterns in your physiology and can compare against long-term trends. Your health data provides sound second opinion and an objective means to share data with a doctor or a coach.