If you sport an activity watch or fitness wearable, it’s likely you are tracking your heart health. Unlike a metronome, your heart does not beat consistently at the same rate. Instead, there are healthy irregularities between the times of each beat. This “variation” in time intervals between heart beats is called your heart rate variability (HRV), and it can tell you a lot about your state of health.
HRV is commonly used by elite athletes as a measure of systemic fatigue and recovery. It can be an indication of overtraining, a reflection of recovery status after a workout, and show how an individual responds to a specific training regimen .
Just like heart rate, HRV fluctuates throughout the day, responding to your body’s stimuli. A decrease in HRV indicates an activation of the sympathetic (or “fight or flight”) branch of your nervous system, which can be caused by activity or stress. An increased HRV shows an activation of the parasympathetic (or “rest and digest”) branch, indicating that you are in a state of recovery.
Paying attention to these patterns in your own HRV can help you personalize your workouts and maximize your athletic potential.
Studies show that training guided and adapted by your HRV is more effective for developing aerobic performance than pre-planned workout regimens that do not account for real-time changes in HRV. [2, 3]. In a study that tested the aerobic performance of individuals during a four-week endurance training period, results showed that those who monitored and adjusted their training regimen according to their HRV had an increased maximal running velocity (load_max) without a significant difference in peak oxygen consumption (VO2_peak), concluding that aerobic performance can be effectively improved by using HRV for a “daily training prescription.” 
So you’ve checked your HRV. Where do you go from here? Use your HRV as a tool to understand how your body responds to certain factors such as:
- Sleep patterns (hours of sleep, sleep quality, wake-up time) 
- Specific workouts (HIIT, running, resistance training, swimming, etc.)
- Hydration 
- Therapeutics (stretching, yoga, meditation) 
- Stress 
By tracking how these factors correlate with your HRV, you can better understand what workouts and methods work best for you. So get out there,and happy tracking!