Heart rate variability (HRV), beat by beat

What is heart rate variability (HRV)?

Heart rate variability, or HRV, is the variation in the time between two successive heartbeats. It's expressed in milliseconds.

Even at 60 beats per minute, your heart doesn't beat exactly once per second: one interval lasts 980 milliseconds, the next 1,030, the third 1,005. This irregularity isn't a flaw. It's the signature of a heart that's constantly adjusting to what your body asks of it.

What HRV reflects: your autonomic nervous system

Two systems steer your heart, without you ever having to think about it:

• The sympathetic system, the accelerator: it takes over in response to stress, exertion or a threat. Your heartbeats become more regular, and HRV drops.
• The parasympathetic system, the brake: carried largely by the vagus nerve, it dominates at rest, during digestion and during recovery. Your heart adjusts beat by beat, and HRV rises.

An HRV that's high relative to your normal signals a body in recovery mode; a low HRV, a body that's still mobilized. The same balance governs your resting heart rate, and your breathing plays a part too: your heart speeds up slightly when you inhale and slows down when you exhale, hence the close link with your respiratory rate.

How HRV is measured: ECG or smartwatch

In the lab, the gold standard is the electrocardiogram (ECG). It detects every heartbeat and measures the intervals, following international standards set back in 1996.

Your watch works differently. Its optical sensor on your wrist uses photoplethysmography (PPG) to detect the pulse wave through your skin. It's less precise than an ECG, but reliable for trends when the watch is worn properly and you're not moving. Nighttime meets both conditions.

Then there's the question of format. Apple Health records HRV as SDNN, the standard deviation of the intervals between heartbeats: that's what the Apple Watch measures. Many studies and other devices use RMSSD, calculated from the differences between successive heartbeats. The two can't be compared directly: a number you read on a forum, or on a friend's watch, tells you nothing about yours.

Why your HRV looks like no one else's

HRV is one of the most individual measurements there is. Depending on age, genetics and fitness, it ranges from around 20 to more than 100 milliseconds. It declines over the years, and endurance training tends to raise it.

The direct consequence: comparing your HRV with a chart you found online makes almost no sense. Two people of the same age, equally fit, can show very different values. The only comparison that counts is you versus you, over several weeks, with the same device.

What lowers HRV, and what helps

• Lack of sleep: a short or broken night leaves your nervous system on alert.
• Alcohol: even in small amounts, a drink in the evening often lowers your HRV the following night.
• Stress: a tense stretch keeps the accelerator pressed down, even at rest.
• Training load: a hard session, or weeks strung together without enough recovery.
• An infection coming on: your body mobilizes its defenses, sometimes before you feel sick.

Conversely, what helps is well known: regular sleep, progressive endurance training, slow breathing, of which resonance breathing is the simplest form, and less alcohol.

How the app reads your HRV

In the app, HRV is the heaviest signal in the vitals pillar of the Readiness Score: 45% of its 42 points, or nearly 19 points out of 100 when all the data is there.

The app reads it from Apple Health, in SDNN format, then compares it with your baseline: your average and standard deviation over the last 42 days, excluding the day being assessed. It takes 7 days of data to get started, and 28 for full confidence. The higher your HRV relative to your normal, the more points it earns; beyond two standard deviations below your average, it earns none at all. The details of all five signals: Vitals.

A low night lowers that day's score, and that's normal. But HRV dropping for one night is noise; a week below your baseline is a signal.

HRV also counts toward your biological age: compared with the values expected for your age, it can move it in either direction, and it counts for less there if you take a medication that slows the heart.

The limits of the measurement

• The optical sensor: a watch that's too loose or an arm that moves, and the reading degrades. Wear it snug, all night.
• The heart rhythm itself: irregular heartbeats, such as premature beats (extrasystoles), throw off the HRV calculation.
• Medications: a medication that slows the heart changes how your HRV should be read.
• Switching devices: changing watches means changing instruments. For the six weeks that follow, your baseline still mixes nights from both devices.

When to see a doctor

HRV isn't a diagnostic tool, and this app isn't a medical device. A low HRV, even several days in a row, isn't a disease: it's an invitation to work on what's pulling it down, to sleep better, to lighten your training.

On the other hand, if a drop settles in for no apparent reason along with unusual fatigue, or if you have palpitations, feel faint, or have chest pain or unusual shortness of breath, see a doctor. In an emergency, call your local emergency number (911 in the US, 999 or 112 in the UK and Europe).