What is HRV (heart rate variability)?

What controls it

Your heart rate is steered by the autonomic nervous system, the part of your nervous system that works without you thinking about it. It has two branches. One gets you ready for action, often called fight or flight. The other, sometimes called the relaxation response, helps you rest and recover.

Harvard Health explains that when the fight or flight side is more active, the variation between beats tends to be lower, and when you’re more relaxed it tends to be higher. That’s why HRV is often used as a rough indicator of stress and recovery.

There’s no single normal number

Harvard Health says there isn’t a recommended HRV value, because HRV varies widely with age, sex, fitness, medical history and genetics. A review of HRV research by Shaffer and Ginsberg notes that HRV tends to decline with age.

How HRV is calculated matters too. Two common methods are SDNN, which looks at the overall spread of the gaps between beats, and RMSSD, which looks at the differences between one gap and the next. Short readings and 24-hour readings aren’t interchangeable either. Apple Health records HRV as SDNN, so a figure from Apple Watch may not match one from a device that uses another method.

What changes your HRV

HRV changes all the time, through the day and from one day to the next. Harvard Health notes that it falls as your heart rate rises, for example during exercise or when you’re under stress, and that chronic stress, poor sleep, lack of exercise and an unhealthy diet can all upset the balance.

Breathing, body position, movement and recent activity also affect a reading, according to Shaffer and Ginsberg. Apple Watch takes its HRV samples while you’re still, so the number of readings you see depends on how active you’ve been.

How to read your own numbers

Look at the trend, not a single reading. Compare today with your own usual over several days or weeks, rather than with a friend or a chart online.

Higher isn’t always better. Shaffer and Ginsberg point out that some health conditions can produce high HRV, and Harvard Health advises against feeling too confident about a high reading or too worried about a low one. It also notes that chest straps tend to measure HRV more accurately than wrist devices, so treat a watch reading as an estimate.

HRV from a watch can’t diagnose anything. The British Heart Foundation advises seeing a GP if your heart rate is fast and you have symptoms such as dizziness, fainting, tiredness or a pounding or fluttering feeling in your chest, and contacting your doctor if you’re worried about your heart rate.

Vital gives general information about fitness and wellbeing. It is not medical advice, and it does not diagnose or treat any condition.