Heart Rate Variability (HRV)

What is heart rate variability?

Heart rate variability (HRV) measures the time variation between consecutive heartbeats, known as R-R intervals. Your heart doesn't beat like a metronome. Even at rest, the gaps between beats constantly shift by milliseconds, driven by your autonomic nervous system (ANS). The sympathetic branch speeds things up; the parasympathetic branch (mainly the vagus nerve) slows them down. HRV captures that tug-of-war in a single number ^[1].

A higher HRV means your body can switch gears quickly between "fight-or-flight" and "rest-and-digest" states. That flexibility signals physiological resilience and has been tied to better cardiovascular health, stronger stress tolerance, and lower all-cause mortality ^[2].

Why heart rate variability matters for longevity

A meta-analysis covering 38,008 people found that low HRV is a significant predictor of higher mortality across all ages ^[3]. Separate research on centenarians showed that those with an SDNN below 19 ms had a 5.7-fold higher risk of dying within one year compared to centenarians with higher values ^[8]. Low HRV is also associated with cardiovascular disease, type 2 diabetes, depression, and chronic inflammation.

HRV naturally declines with age, but the rate of decline differs. Parasympathetic HRV markers tend to drop until around the 7th or 8th decade of life, then stabilize or even slightly rise in the longest-lived individuals. Lifelong endurance athletes often maintain RMSSD values 20-30% above their age-matched peers ^[5]. This suggests that HRV isn't just a passive biomarker but something you can actively protect through lifestyle choices.

How to measure heart rate variability

The gold standard is electrocardiography (ECG), which directly reads the heart's electrical signals. Most consumer wearables (Apple Watch, Oura Ring, Garmin, WHOOP) use optical photoplethysmography (PPG) instead, measuring blood-flow changes at the wrist or finger. PPG is convenient for daily tracking but less precise than ECG chest straps, especially during movement ^[4].

Nearly all wearables report RMSSD (root mean square of successive differences), the standard time-domain metric for short-term parasympathetic activity. Measurements taken during sleep are the most reliable because body position, breathing, and mental state are relatively stable. For meaningful trends, measure at the same time each day, ideally first thing in the morning or during overnight sleep.

What is a good heart rate variability score?

Normal HRV values vary widely by age, sex, and fitness level. A systematic review of 57 studies found average RMSSD values around 42-60 ms in adults under 30, dropping to roughly 20-30 ms by age 60 and beyond ^[5]. Women aged 20-45 tend to have RMSSD values about 5 ms higher than men of the same age; the difference narrows after 45 and disappears after 60.

Population averages are useful context, but your personal baseline matters more. Track your own 7-day rolling average. A consistent drop of 10-15% below your baseline over several days may signal overtraining, accumulated stress, sleep debt, or an oncoming illness. Single-day dips are usually noise.

What causes low heart rate variability?

Chronically low HRV reflects a nervous system stuck in sympathetic dominance. Common drivers include poor or short sleep, chronic psychological stress, physical inactivity, excessive alcohol intake, and underlying conditions like hypertension, diabetes, or heart failure. Alcohol is particularly instructive: even two standard drinks suppress HRV by 28-33% for hours, primarily by blunting vagal tone ^[9]. A single drink, by contrast, has minimal impact.

Aging itself lowers HRV, but sedentary aging accelerates the decline far more than active aging does. This is good news because it means much of the age-related drop is modifiable.

How to improve heart rate variability

The strongest evidence points to aerobic exercise. A 2024 meta-analysis of 16 randomized controlled trials found that regular aerobic training improved RMSSD by a standardized mean difference of 0.84, with gains appearing within 8-12 weeks ^[6]. Zone 2 training (low-intensity steady-state cardio) appears particularly effective for building parasympathetic tone without triggering excessive sympathetic stress.

Slow breathing at around 6 breaths per minute (inhale for 5 seconds, exhale for 5 seconds) activates the vagus nerve and stimulates the baroreflex. A meta-analysis confirmed that this breathing pattern acutely increases HRV, and regular practice (10-20 minutes daily) can produce lasting improvements over weeks ^[7]. HRV biofeedback, which combines slow breathing with real-time HRV feedback, has shown medium effect sizes for reducing depression and anxiety while boosting vagal tone ^[10].

Other evidence-based strategies include consistent sleep of 7-9 hours, omega-3 fatty acid supplementation (1-2g EPA/DHA daily), cold-water immersion after exercise (which accelerates parasympathetic reactivation ^[11]), and stress reduction through meditation or time in nature. No single intervention works best on its own. The most durable improvements come from stacking several of these habits consistently.