Heart rate variability (HRV)

What heart rate variability is

Heart rate variability (HRV) is the variation in the time between one heartbeat and the next. A heart at 60 beats per minute does not tick once a second: one gap is 980 ms, the next 1 050 ms, the one after 1 010 ms. That beat-to-beat jitter is HRV, and it is measured in milliseconds, not in beats.

The jitter is not noise. It is the nervous system steering the heart moment by moment: a touch faster on the in-breath, a touch slower on the out-breath, an adjustment for posture, temperature and what you just thought about. A heart that can be steered that finely belongs to a body with reserves. A heart that beats like a metronome is usually a body under load, holding one setting because it cannot afford to shift.

Why a higher number is usually better

HRV is the most direct everyday window into the autonomic nervous system, the part that runs without you. Its two branches pull in opposite directions: the parasympathetic branch slows the heart and repairs, the sympathetic branch speeds it up and mobilises. Overnight HRV rises when the parasympathetic branch is in charge, which is what a recovered body looks like, and falls when the sympathetic branch stays on, which is what stress, illness, alcohol and a hard training day look like from the inside.

That is why a single number can respond to so many different things. A short night, a late drink, a fever coming on, a week of deadlines and an interval session all push the same lever in the same direction. HRV cannot tell you which one did it; it tells you that the system is loaded, and your calendar usually tells you why.

How wearables measure it

Consumer devices measure HRV from the pulse wave at the wrist or finger, using the same optical sensor that gives heart rate. Because movement and daylight disturb the signal, nearly all of them take the reading during sleep, when the body is still and the conditions repeat night after night. Garmin, Polar, Oura, Fitbit, Withings and Apple all publish some form of overnight or resting HRV; a chest strap such as the Polar H10 reads the electrical signal directly and is the reference for short daytime measurements.

The statistic most of them report is RMSSD, the root mean square of successive differences between beats. It tracks the parasympathetic branch well, it works on recordings of a few minutes, and it is the figure research papers usually mean by "HRV". Daytime stress scores on a watch come from the same signal, sampled in shorter windows and inverted, so a high stress reading and a low HRV reading are two views of one measurement.

One thing the manufacturers do not share is the recipe. Each brand filters the signal, picks the windows and averages the night its own way, so the same person wearing two devices gets two different numbers. Across MyBodyAI accounts the nightly means from Fitbit sit around 25 ms, Polar and Oura around 35 to 47 ms, Garmin near 70 ms and Withings above 100 ms. None of them is wrong; they are different instruments. It is the reason a number from a friend's watch means nothing for yours.

What is a good HRV

There is no threshold that separates healthy from unhealthy. HRV falls with age, is higher in athletes, differs between sexes and, as above, differs between devices. A 25-year-old runner may see an RMSSD of 80 to 120 ms; a 55-year-old with an office job may see 15 to 30 ms. Both can be perfectly well. Published reference values (Nunan 2010, Shaffer and Ginsberg 2017) come from short daytime recordings pooled across studies, not from nightly readings by age, so they cannot place your overnight number either.

What carries information is your own baseline and your distance from it. A night 20 % under your usual value after a hard session is expected and passes; the same 20 % holding for a week with no training to explain it is the signal worth acting on. The direction of the trend matters more than any single reading, and a single reading matters more than none.

How to raise it

The levers with the strongest evidence, in roughly that order:

• Sleep, regular and enough. A consistent bedtime within about half an hour and seven to nine hours in bed do more for overnight HRV than any supplement.
• Less alcohol. Even a modest evening dose lowers the following night's HRV and raises resting heart rate; it is the most reliable way to produce a bad reading on demand.
• Easy aerobic work. Months of zone 2 training raise resting HRV; the effect is slow and it comes from volume at conversational pace, not from intensity.
• Slow breathing. Breathing at about six breaths a minute for five to ten minutes raises HRV during and shortly after the session (Lehrer and Gevirtz 2014); regular practice is associated with higher resting values.
• Recovery after hard days. HRV drops the morning after a demanding session and returns within a day or two when the load was right. If it stays down, the load was not.

HRV in MyBodyAI

MyBodyAI reads the nightly HRV your device already computes: Polar, Fitbit, Withings and Oura through their connections, Garmin through the data export. The Garmin watch app adds a separate morning measurement taken right after waking, stored beside the night rather than mixed into it. The app never invents a nightly value from daytime samples.

The number feeds five of the twelve health indices: Nervous System, Recovery, Immunity, Body Alert and Cardiac Fitness. Nervous System, Recovery and Immunity score the reading two ways at once: against your own thirty-day baseline, which is the trend, and on an absolute ladder that shifts to where your readings usually sit once the app has seven nights from you. Body Alert watches for a fall against that baseline, and Cardiac Fitness reads the last week's level beside your resting heart rate. The shift is what lets a Fitbit wearer at 26 ms and a Withings wearer at 105 ms both score in the right place; there is no age table behind it, because none exists for nightly readings.

For a first orientation without an account, the HRV Decoder takes a reading and your age and says roughly where it sits. The app itself does not stop there: it works from your own nights.