Heart rate variability (HRV), explained

Take your pulse and you get a number: 62 beats per minute, say. It suggests a heart ticking at a fixed interval. In reality, the gap between beats shifts every time, by a fraction of a second. That micro-variation is your heart rate variability, or HRV.

Far from a flaw, it is a sign of health. A fit heart is a flexible heart, constantly adjusting its rhythm.

#What HRV actually measures

HRV is a window into your autonomic nervous system, the part that runs your heart, breathing and digestion without your input. It has two branches:

• The sympathetic, the accelerator, which takes over under stress or effort.
• The parasympathetic, the brake, carried by the vagus nerve, which dominates at rest and during recovery.

With every breath, these two branches gently trade control: the heart speeds up a little on the inhale, slows on the exhale. The wider that dance, the higher your HRV, and the more responsive and balanced your nervous system. This is the same mechanism coherent breathing exploits by slowing the breath.

#High vs low HRV: how to read it

In general:

• High HRV reflects strong parasympathetic tone: you recover well and absorb stress well.
• Low HRV signals a system under load: fatigue, chronic stress, a short night, oncoming illness, or overtraining.

But beware the absolute-number trap. HRV varies enormously from person to person by age, genetics and fitness. Comparing your HRV to a friend's or to an average is meaningless. What matters is your own trend over time.

#RMSSD, the metric to remember

There are several ways to compute HRV. For day-to-day tracking, the most useful is RMSSD (root mean square of successive differences, if you want the full name). What matters is what it represents: vagus nerve activity, and therefore your recovery capacity right now. It is the metric most apps and watches track.

#How to measure it

Three levels of precision:

1. A chest strap (a Bluetooth heart rate sensor). The most reliable for HRV, because it reads each beat precisely. This is what Resonance uses for its test.
2. A smartwatch or smart ring. Handy for an overnight trend, a little less precise for a single spot reading.
3. The phone camera (fingertip pulse reading). Fine for a ballpark, less so for fine tracking.

For a reading you can compare day to day, always take it in the same conditions: in the morning, calm, in a steady position, a few minutes after waking. A reading taken after coffee or mid-rush tells you nothing.

#HRV training: how to raise it

HRV responds to lifestyle, not gadgets. The levers that work:

• Sleep. By far the biggest factor. Regular, sufficient nights raise HRV.
• Physical activity, in reasonable doses. Too much training without recovery drives it down instead, which is why athletes track it.
• Slow breathing. Coherent breathing is one of the few fast-acting tools. This is what people mean by "HRV training": a few weeks of daily practice often lifts resting HRV by strengthening vagal tone.
• Less alcohol and fewer late meals, which both weigh on overnight recovery.

The mechanism behind HRV biofeedback, and why breathing at your resonance frequency works, is laid out in the review by Lehrer and Gevirtz (Frontiers in Psychology, 2014).

#Using HRV well

Take HRV for what it is: a compass, not a verdict. A low number one morning does not mean your day is ruined; it just invites you to ease off. What is instructive is the trend over several weeks. If it climbs steadily as you sleep better and practice the 3-6-5 routine, that is concrete evidence your nervous system is gaining flexibility.

And if a low trend persists with no explanation, talk to a health professional. HRV is a useful indicator, not a diagnosis.