After losing two family members to cancer, Salim Najjar dedicated his life to understanding how our bodies process stress. After years of research, his favorite metric to track became heart rate variability-- the variance between each heartbeat.
While most of us think about heart rate (beats per minute), HRV measures the fluctuations in intervals between your heart beats, which reveals how adaptable your nervous system is to stress. The higher your HRV, the more resilient you are, and the more bandwidth you have.
The Optimist