HRV: what heart rate variability measures, what it predicts, and how to read your own

What it means

Heart rate variability is the variation in time between consecutive heartbeats, measured in milliseconds. A healthy heart does not beat like a metronome; the parasympathetic (vagal) nervous system constantly nudges the interval, and more variation means more vagal tone and more capacity to respond to stress. The common metric is RMSSD, usually measured overnight or on waking.

HRV falls with age, with stress, illness, alcohol and overtraining, and rises with fitness, sleep and recovery. Normal values vary enormously between people (20–100 ms is all "normal"), which is why only your own trend is meaningful.

Why it matters for longevity

Low HRV predicts cardiovascular events and all-cause mortality independently of other risk factors, and it is the earliest signal of overtraining, illness and HPA-axis strain. For a training plan, it answers the daily question — hard session or easy day — better than how you feel. For a longevity plan, its trend over months reflects whether the whole system is adapting or wearing down.

What the evidence says

Framingham (Tsuji 1996) and ARIC (Dekker 2000) linked low HRV to cardiac events and mortality; a 2021 meta-analysis confirmed the association across 30 cohorts. In athletes, HRV-guided training — doing hard sessions only when HRV is at or above baseline — produced better endurance gains than fixed schedules in randomised trials (Kiviniemi 2007, Vesterinen 2016).

What raises it with evidence: endurance training (the largest effect), sleep, weight loss, slow breathing at about six breaths per minute, and no alcohol. A single drink lowers next-morning HRV measurably; a hard interval session lowers it for 24–48 hours.

How to measure or use it

• A chest strap or a validated wearable, measured in the same state each day — overnight or on waking, lying still. Use the seven-day rolling average, not the daily value.
• Read it as a traffic light against your own 60-day baseline: within the normal band, train as planned; well below for two or more days, go easy and look for the cause (sleep, alcohol, illness, load).
• Over months, a rising baseline means the plan is working; a falling one means too much load or too little sleep.

Related

• Sleep and longevity — HRV as the sleep readout
• VO2max and longevity — HRV-guided training in the plan
• Biomarkers of ageing — where HRV sits among the markers