Heart Rate Variability (HRV)

What it is

HRV is the variation between successive heartbeats (R-R intervals). Higher HRV — especially the high-frequency / parasympathetic component — reflects a flexible, well-regulated autonomic nervous system. Commonly reported metrics:

• RMSSD — root mean square of successive differences (parasympathetic emphasis).
• SDNN — standard deviation of normal R-R intervals (overall variability).
• LF/HF ratio — frequency-domain estimate of sympathetic/parasympathetic balance (interpretation debated).

Why it matters

Lower HRV in middle-aged adults predicts cardiovascular events and all-cause mortality. Acute drops in HRV mark over-training, illness, stress, or sleep deprivation. Athletes use HRV to guide training load and recovery.

Reference and trend

Absolute values vary widely between individuals (genetic and methodological factors). The useful signal is your own trend over time and against your personal baseline.

What raises HRV

• Exercise — both endurance and resistance.
• Adequate sleep.
• Reducing alcohol intake.
• Stress management (breathing, meditation, time outdoors).
• Heat exposure (sauna).
• Improving cardiometabolic risk factors.

How to measure

• Chest-strap during stillness (most accurate).
• Optical sensors in smartwatches and smart rings (Oura, WHOOP, Garmin) — good enough for trend tracking.

Related entries

Resting heart rate, VO2max, Sleep optimization.