Heart rate variability (HRV): what it is, what moves it and how to read a band

What is heart rate variability?

Your heart does not tick like a metronome. Even at a steady 60 beats a minute, the gaps between beats are not all exactly one second. Heart rate variability (HRV) is the measure of how much those gaps vary.

A 2024 narrative review in Frontiers in Physiology by Sammito and colleagues defines HRV through the variation between consecutive intervals between normal beats, called NN intervals. It says this variability depends on sympathetic and parasympathetic influences, the two arms of the autonomic nervous system.¹ The same review notes that HRV can be calculated in several ways, from time-based, frequency-based and non-linear methods, and that these do not all move in the same direction: for some measures a higher value means more variability, for others the reverse.¹

That last point is why two apps can show different HRV numbers for the same night. They may be using different calculations, over different time windows.

Is there a normal HRV number?

This guide does not give a table of normal HRV values in milliseconds. None of the British sources read for it, the NHS or the British Heart Foundation (BHF), publishes one, and the review it relies on says that comparing different people's HRV is affected by many confounders.¹³ A figure copied from a chart for another person's age, sex and device would mislead more than it informed.

What the review does say is useful in a different way. Intra-individual comparisons, meaning your own readings compared with your own earlier ones, are usually uncomplicated, while comparisons between people run into the confounders listed below.¹ For a home user, that is the whole method: learn your own baseline and watch for changes.

What changes heart rate variability?

Age and sex

You cannot change your age, and the review says HRV generally falls with it.¹ That is a reason not to compare your HRV with a friend's, or with your own at 25.

Night and morning

Because HRV is higher during the night and drops in the morning, the review says short measurements can only be compared meaningfully at the same time of day.¹ A band that records overnight has an advantage here, since it takes readings in similar conditions each night.

Lifestyle

Alcohol, smoking and stress all pull the figure down in the review.¹ The BHF makes a related point about heart rate itself: alcohol can temporarily increase it and make the heart feel as if it is pounding or fluttering.⁴ A night after drinks is therefore a common explanation for a low HRV reading, not a mystery.

What does a wellness band measure, and how well?

Wrist bands estimate HRV from light sensors that detect blood flow, not from an electrocardiogram (ECG). The BHF says wrist-worn monitors use light sensors to measure blood flow, and that readings can be affected by how tightly the device fits, movement, sweat, temperature and lighting.³

A 2025 study in Physiological Reports tested nocturnal resting heart rate and HRV from five consumer wearables against an ECG reference. Thirteen healthy adults wore them over 536 nights. The authors report that accuracy varied significantly between devices. For HRV, the mean absolute percentage error ranged from 5.96% for the best-performing device to 16.32% for the weakest.²

Two practical readings follow. First, a few percent of error is expected even from a well-tested device, so small day-to-day wobbles are not meaningful. Second, a change of the kind that persists for several days, in the same direction, is more informative than any single value.

A worked example

Imagine a band reports these invented nightly HRV figures in milliseconds over a week: 54, 50, 56, 52, 49, 55 and 53. They add up to 369, and 369 divided by 7 gives an average of about 53. Now imagine the next week reads 44, 47, 45, 46, 48, 43 and 45. That sums to 318, an average of about 45. The week-on-week drop is about 7 ms, or roughly 14% of the first week's average. This is the kind of sustained shift worth asking about: did you drink more, sleep less, start a new medicine, or catch a cold? These numbers are made up to show the arithmetic. They are not typical values.

What can HRV not tell you?

The review notes that low HRV has been linked with higher mortality in people with conditions such as a past heart attack, stroke or heart failure.¹ That is a finding about groups in clinical research, using careful measurement. It does not mean a low reading on a wrist band says anything about your own heart.

The BHF is direct about consumer devices. It says they can help you spot patterns or changes, but they cannot diagnose a heart problem or give a full picture of your heart health.³ It also says that if a monitor leaves you worried about your heart rate, you should speak to your GP rather than trying to work out what the results mean yourself.³

So an HRV figure is a wellness indicator. It can prompt you to look at sleep, alcohol, stress and training. It cannot tell you whether you have a heart rhythm problem.

For the heart rate side of the picture, see the guide to normal resting heart rate. For how nights feed into recovery, see how much sleep you need.

When should you speak to a health professional?

Get advice about your heart, not about your HRV number. The NHS says to see a GP if palpitations keep coming back or are happening more often, last longer than a few minutes, or if you have a heart condition or a family history of heart problems.⁵

The NHS says to call 999 or go to A&E if you have palpitations that do not go away, or you also have chest pain, shortness of breath, or feel faint or like fainting.⁵ If those symptoms have stopped, ask your GP surgery for an urgent appointment, call 111 or use 111 online.⁵

Frequently asked questions