An HRV score measures the variation in time between consecutive heartbeats, expressed in milliseconds. If your heart beats 60 times per minute, that doesn’t mean each beat lands exactly one second apart. The gaps between beats constantly shift, sometimes 0.9 seconds, sometimes 1.1 seconds. That variation is your heart rate variability, and it reflects how well your nervous system adapts to changing demands. A higher HRV generally signals a more resilient, adaptable body. A lower HRV suggests your system is under strain.
How Your Nervous System Controls HRV
Your heart has its own built-in pacemaker, but the timing of each beat is largely controlled by two competing branches of your autonomic nervous system. The parasympathetic branch (sometimes called “rest and digest”) slows your heart through the vagus nerve. The sympathetic branch (“fight or flight”) speeds it up by releasing adrenaline and noradrenaline. These two systems are constantly pushing and pulling against each other, and that tug-of-war creates the beat-to-beat variation in timing.
At rest, the calming parasympathetic side dominates. The vagus nerve releases a chemical messenger that slows the heart’s pacemaker cells, and this influence fluctuates with your breathing, blood pressure changes, and signals from the brain. When you inhale, your heart rate briefly speeds up. When you exhale, it slows down. This rhythmic fluctuation is a sign that your nervous system is responsive and flexible, which is exactly what a high HRV reflects.
When stress, illness, or fatigue tips the balance toward constant sympathetic activation, the heart beats faster and more rigidly. There’s less room for variation between beats, and HRV drops.
How HRV Is Calculated
Most wearable devices measure HRV using a metric called RMSSD, which stands for “root mean square of successive differences.” In practical terms, the device records the time gap between each heartbeat, calculates how much each gap differs from the one before it, and produces a single number in milliseconds. RMSSD captures short-term, beat-to-beat changes, making it a direct reflection of parasympathetic activity. It’s the most commonly used HRV metric because it’s straightforward to compute and works well with short recording windows, like overnight sleep.
Another common metric is SDNN, which measures the overall spread of all beat-to-beat intervals over a recording period. SDNN captures both short-term and longer-term variability, making it a broader indicator but one that’s more sensitive to recording length. A 5-minute SDNN reading and a 24-hour SDNN reading aren’t comparable. Most consumer wearables favor RMSSD for this reason.
Some devices translate these raw millisecond values into a proprietary 0-100 scale, which can make comparisons between brands confusing. If your watch gives you a score of 72, that number may not mean the same thing on a different brand’s device. Knowing whether your device reports raw RMSSD or a converted score helps you interpret what you’re seeing.
Normal HRV Ranges by Age
HRV declines naturally with age. A 25-year-old and a 65-year-old with identical fitness levels will have meaningfully different scores. The table below shows RMSSD values in milliseconds across age groups, with the 10th percentile representing the low end, the 50th as the median, and the 90th as the high end.
- Ages 18–24: Median around 46–48 ms, with the top 10% reaching 82–85 ms
- Ages 25–34: Median around 41–43 ms, top 10% at 73–77 ms
- Ages 35–44: Median around 34–36 ms, top 10% at 60–66 ms
- Ages 45–54: Median around 28–30 ms, top 10% at 52–58 ms
- Ages 55–64: Median around 24–25 ms, top 10% at 46–50 ms
- Ages 65–74: Median around 22–23 ms, top 10% at 40–44 ms
- Ages 75+: Median around 20 ms, top 10% at 36–39 ms
Men and women show similar values, with women trending slightly higher in younger age groups and slightly lower in older ones. The differences between sexes are small compared to the differences between age groups. What matters most isn’t where you fall on a population chart but how your own score trends over weeks and months.
What a Low HRV Score Means
Low HRV indicates that your body is less able to flexibly shift between states of alertness and recovery. In the short term, a dip in your score can reflect poor sleep, acute stress, illness, or a hard workout you haven’t recovered from. These temporary drops are normal and expected.
Chronically low HRV is a different story. A large population study published in Circulation found that middle-aged adults with persistently low HRV had a higher risk of coronary heart disease and death from all causes, including cancer. Low HRV has also been documented in people with diabetes, high blood pressure, thickened heart muscle, atherosclerosis, obstructive sleep apnea, and depression. In fact, reduced HRV has long been used as an early marker for nerve damage caused by diabetes.
Several factors drive HRV downward over time: physical inactivity, smoking, shallow breathing patterns, high insulin levels, and chronic psychological stress. Many of these are modifiable, which is what makes tracking HRV useful in the first place.
What Affects Your Score Day to Day
Alcohol has one of the most measurable effects on overnight HRV. A large study of Finnish employees wearing heart rate monitors found that even low alcohol intake reduced RMSSD by about 2 ms and raised resting heart rate by 1.4 bpm during sleep. Moderate intake dropped RMSSD by 5.7 ms and raised heart rate by 4 bpm. Heavy drinking cut RMSSD by nearly 13 ms, raised heart rate by close to 9 bpm, and reduced the body’s physiological recovery during sleep by roughly 39 percentage points. On nights without alcohol, recovery improved progressively as sleep continued. After heavy drinking, that progressive recovery simply didn’t happen.
Sleep quality plays a central role. During deep sleep, your parasympathetic system dominates and your body shifts into its most restorative state. When deep sleep is cut short or fragmented, that window of recovery shrinks. Poor sleep blunts the nervous system’s ability to relax, disrupts metabolic processes, and reliably lowers HRV the next morning.
Acute stress, whether from work pressure, emotional conflict, or overtraining, shifts the nervous system toward sympathetic dominance. Your body stays in a vigilant state even during sleep, and HRV reflects that. One stressful day won’t permanently change your score, but weeks of unmanaged stress create a noticeable downward trend.
Using HRV to Guide Training and Recovery
For athletes and regular exercisers, HRV works best as a readiness indicator. The idea is simple: measure your HRV each morning, compare it to your personal baseline, and adjust your plans accordingly. A score above your baseline suggests your body has recovered and can handle intense training. A score below baseline signals that your system is still recovering, and a lighter workout or rest day would be more productive.
Building a reliable baseline takes about a week of consistent measurements. Most wearables do this automatically, recording HRV during sleep and averaging it over a rolling window. Once you have that reference point, the daily fluctuations become meaningful. A single low reading isn’t cause for alarm. A string of below-baseline readings over several days suggests accumulated fatigue, insufficient sleep, or rising stress.
The key principle is that your individual trend matters far more than any absolute number. Someone with a baseline of 30 ms who sees their score rise to 38 ms is showing the same positive adaptation as someone whose baseline of 55 ms rises to 65 ms. Comparing your number to someone else’s is rarely useful.
How to Improve Your HRV
Because HRV reflects the overall state of your autonomic nervous system, improving it generally means reducing the load on your sympathetic “fight or flight” response and strengthening parasympathetic tone. The most effective levers are the familiar ones: consistent aerobic exercise, quality sleep, stress management, and moderating alcohol intake.
Regular cardiovascular exercise is one of the strongest drivers of higher HRV over time. It doesn’t need to be extreme. Consistent moderate activity, like brisk walking, cycling, or swimming several times a week, builds the cardiovascular fitness that supports greater beat-to-beat variability. Overtraining, however, does the opposite. Pushing too hard without adequate recovery suppresses HRV, which is precisely why athletes use it as a recovery gauge.
Slow, deep breathing directly activates the vagus nerve and shifts the nervous system toward parasympathetic dominance. Even a few minutes of deliberate breathing at roughly six breaths per minute can produce a measurable increase in HRV during the session. Over time, regular breathwork or meditation practice appears to raise baseline HRV, though the effect builds gradually.
Protecting sleep is arguably the highest-impact change. Going to bed at a consistent time, limiting alcohol in the hours before sleep, and keeping your bedroom cool and dark all support the deep sleep stages where parasympathetic recovery is strongest. Given that heavy drinking alone can erase nearly 40% of your body’s overnight recovery, cutting back on evening alcohol is one of the fastest ways to see your morning HRV scores improve.