What Is HRV? Heart Rate Variability Explained, with Normal Ranges by Age

Why does the heart vary at all?

Your autonomic nervous system has two branches. The sympathetic branch (fight or flight) speeds the heart up and keeps it steady. The parasympathetic branch (rest and digest), acting through the vagus nerve, slows it and lets it fluctuate with your breathing. When you are recovered and relaxed, the vagus nerve dominates and HRV is high. When you are stressed, ill, sleep-deprived or over-trained, the sympathetic branch takes over and HRV falls.

The main HRV metrics

Metric	What it measures	Use it for
RMSSD	Root mean square of successive beat-to-beat differences	The standard for short (1 to 5 minute) readings and daily readiness. This is the headline HRV number in Pulse Tracker.
SDNN	Standard deviation of all beat intervals	Overall variability. Most meaningful over 24 hours, but useful as a secondary short-term metric.
pNN50	Percentage of successive intervals differing by more than 50 ms	Another measure of vagal activity; falls to near 0 in highly stressed states.
Mean R-R	Average time between beats, in ms	Simply 60,000 divided by heart rate. 1000 ms = 60 BPM.

Normal HRV by age

HRV declines with age and varies enormously between individuals, so any table is a rough guide only. These RMSSD ranges are from short resting measurements in healthy adults.

Age	Typical RMSSD (ms)
20 to 29	40 to 100
30 to 39	35 to 80
40 to 49	30 to 65
50 to 59	25 to 55
60 and over	20 to 50

What lowers HRV

• Short or poor-quality sleep
• Alcohol, even moderate amounts, for 24 to 48 hours
• Illness, often a day or two before symptoms
• Hard training without enough recovery
• Psychological stress and anxiety
• Dehydration, heat and late heavy meals
• Stimulants and some medications

How to improve HRV

1. Sleep: regular timing, 7 to 9 hours, dark cool room.
2. Aerobic exercise: consistent moderate cardio raises resting HRV over weeks.
3. Slow breathing: 5 to 6 breaths a minute for 10 minutes raises HRV immediately and, practised daily, over time. Pulse Tracker includes resonance breathing for exactly this.
4. Less alcohol, especially in the evening.
5. Manage stress: meditation, time outdoors, social connection.
6. Hydration and nutrition: avoid large late meals and chronic under-fuelling.

How to measure HRV with an iPhone

You don't need a chest strap. Open Pulse Tracker, place your fingertip on the rear camera and stay still for 30 seconds. The app detects each pulse, measures the intervals and reports RMSSD, SDNN, pNN50 and your stress and energy levels. Measure every morning before coffee for a trend you can act on.

Related: Normal resting heart rate · Stress and energy monitor · HRV app

Wellness disclaimer. Pulse Tracker is intended for general wellness use only. It is not a medical device, is not FDA cleared or approved, and is not intended to diagnose, treat, cure or prevent any disease or condition. Blood pressure shown in the app is either a value you entered from your own cuff or an estimate derived from the pulse signal after cuff calibration; it is not a measurement and must not be used to make medical decisions. Read the full wellness disclaimer. Camera readings, estimates and classifications are for general information only and can be affected by movement, lighting, skin temperature and other factors. Do not use them to diagnose, treat or make decisions about a medical condition. Always consult a qualified healthcare professional, and call emergency services if you have chest pain, shortness of breath, fainting or other urgent symptoms.