What is HRV, and what is a good score?

What HRV actually measures

Your heart does not beat like a metronome. Even at complete rest, the gap between one beat and the next varies by a few milliseconds each time. That variation is not a fault. It is the visible output of your autonomic nervous system continuously adjusting your heart rate in response to breathing, blood pressure, temperature, hormones, and stress.

Two branches compete for control. The sympathetic branch drives the fight-or-flight response: it raises heart rate and reduces variability. The parasympathetic branch, carried largely by the vagus nerve, does the opposite: it slows the heart and increases variability. When you are well recovered, the parasympathetic side has more influence and your beat-to-beat variation goes up. When you are stressed, sick, hungover, or deep in a training block, the sympathetic side dominates and variability falls.

Apple Watch reports HRV as SDNN, the standard deviation of the intervals between normal heartbeats, in milliseconds. It samples this periodically, and the readings that matter most for recovery are the ones taken overnight while you sleep, when external influences like posture, caffeine, and movement are largely absent.

What counts as a good HRV score

This is the question everyone asks, and the honest answer is that there is no universal good number. Published overnight SDNN values for healthy adults span roughly 20 to 200 ms. A 25-year-old endurance athlete might sit at 120 ms; a healthy 55-year-old might sit at 35 ms. Neither is unwell.

Three things drive most of that spread:

• Age. HRV declines steadily across adulthood. This is normal and expected.
• Genetics. A large share of the between-person difference is simply inherited.
• Aerobic fitness. Endurance-trained people tend to sit higher, though the effect is smaller than most people assume.

Because of this, comparing your HRV to a friend's, or to a population chart, tells you almost nothing. The useful question is not is my HRV high? but is my HRV where it usually is? A 20 % drop below your own two-week baseline is informative regardless of whether your baseline is 35 ms or 135 ms.

What lowers your HRV

These are the reliable, well-documented suppressors, roughly in order of how sharply they tend to bite:

• Alcohol. Among the most consistent and dramatic acute effects. Even a couple of drinks can flatten overnight HRV, and the effect is largest in the first half of the night.
• Short or fragmented sleep. Both the duration and the continuity matter.
• Hard or unaccustomed training. Expected, and not a problem on its own. It becomes a problem when it does not rebound within a day or two.
• Illness. HRV often falls a day or more before you consciously feel unwell, which is why it is worth watching alongside resting heart rate and temperature.
• Psychological stress. A demanding week at work registers here as clearly as a hard training week.
• Late meals, dehydration, and heat. Smaller, but real, and they stack.

How to actually use it

Single readings are noisy. Use the trend.

1. Establish a baseline. Wear your watch overnight for at least a week, ideally two, before drawing any conclusions.
2. Watch the deviation, not the number. A meaningful signal is your reading sitting clearly outside its usual range, not moving a few milliseconds.
3. Corroborate before you act. One low morning is noise. A low reading alongside an elevated resting heart rate, a warmer wrist temperature, and poor sleep is a pattern worth respecting.
4. Do not chase it. HRV is a readout, not a target. Training specifically to raise a number tends to produce worse decisions than simply sleeping well and training sensibly.

This is exactly how Apex Health: Recovery Coach uses it: HRV is weighted against an exponentially-weighted baseline that excludes the current day, so today's reading is judged against your recent normal rather than dragged toward itself.