Heart Rate Variability measures the variation in time between heartbeats. Higher HRV indicates greater autonomic nervous system flexibility and is associated with resilience, fitness, and lower mortality risk.
Root Mean Square of Successive Differences — the gold-standard time-domain HRV metric. Reflects parasympathetic (vagal) tone. Healthy adults: 25–100ms. Elite athletes may exceed 120ms.
Even moderate alcohol (2+ drinks) suppresses HRV for 24–48 hours, reducing parasympathetic tone by 15–30%. This effect persists well past subjective intoxication.
2–3 minute cold water immersion (10–15°C) activates the dive reflex, acutely elevating vagal tone. Regular practice shows cumulative HRV improvements of 8–15% over 4 weeks.
