They flash up reassuring figures about your heart rate, sleep patterns and daily steps – and may even warn if your blood pressure is dangerously high.
But new research shows relying on certain smartwatches to track your blood pressure could leave millions falsely believing they are in the clear, potentially leaving them at increased risk of a heart attack or stroke.
Around 16 million people in the UK are known to have high blood pressure (or hypertension); another five million may be affected without realising, as it can be symptomless. However, if the force of blood pushing against artery walls is consistently high, it puts strain on the heart and blood vessels – increasing the risk of heart attack and stroke.
Some smartwatches promise to help spot the problem.
They don’t measure blood pressure in the same way as the cuff used by a GP or pharmacist, which inflates around the upper arm to measure pressure in the artery.
Instead, they use sensors to pick up the pulse at the wrist – an algorithm uses this information to estimate the blood pressure.
This estimate can, however, be affected by movement, or the position of the arm or watch.
Yet millions rely on them. Blood pressure-monitoring smart devices include the Apple Watch – which is used by 200 million people globally – while those made by Huawei, Samsung and Xiaomi have more than 400 million users. A version of the Apple Watch launched a year ago had its ‘hypertension notification feature’ approved by the US Food and Drug Administration. It features an optical sensor which uses light to track the pulse at the wrist.
High blood pressure changes the way that pulse travels through the arteries, so the watch looks for those changes over 30 days, building up an average reading.
Blood pressure-monitoring smart devices include the Apple Watch – which is used by 200 million people globally
But recent research in the journal JAMA found worrying evidence that it failed to detect more than half of those who had undiagnosed high blood pressure. (The NHS considers blood pressure high at 135/85 or above when measured at home. An ideal reading is between 90/60 and 120/80.)
Experts at the University of Pennsylvania analysed Apple’s test data involving 1,800 volunteers to determine how often it got readings right or wrong. They then combined that with separate US data from another 4,000 adults, to work out how many with undiagnosed high blood pressure would be likely to receive a warning if the watch was used more widely.
The results suggested that only 41 per cent with hidden high blood pressure received an Apple Watch alert.
The device did better in those who had healthy blood pressure but still made mistakes.
In 92 per cent of these cases, there was – correctly – no alert, but 7 per cent were warned their blood pressure was high (when 30 days of monitoring with a proper cuff had already confirmed it wasn’t). Apple said the feature is ‘not intended to diagnose, treat or aid in the management of hypertension or other conditions’.
This study follows a warning last year from the American Heart Association that smartwatches and other wearables designed to track blood pressure are not yet reliable enough to diagnose the condition.
‘Cuffless blood pressure devices are convenient and capable of frequent or continuous monitoring,’ says Dr Jordana Cohen, a hypertension consultant at the University of Pennsylvania in the US, who was part of the American Heart Association review.
‘However, the speed of commercialisation has outpaced the science. Without proper validation, readings from cuffless blood pressure devices are not reliable enough to inform treatment decisions.’
Similar issues have emerged with smartwatch alerts for abnormal heart rhythms.
A 2020 US study in the Journal of the American Medical Informatics Association examined the outcome of 264 people who sought medical help after their Apple Watch detected an abnormal heartbeat.
The most common type of heart rhythm problem, atrial fibrillation (AF) – an irregular heartbeat – can allow blood to pool in the heart, forming clots and increasing the risk of stroke.
Just 30 of the 264 in the study – 11.4 per cent – were subsequently found to have a heart problem that required medical action.
This is because there are limits to the electrocardiogram (ECG) function built into smartwatches to measure heart rate and the heart’s electrical activity.
ECG machines used in GP surgeries and hospitals take 12 different electrical measurements, giving doctors a detailed picture.
‘Most ECGs in smartwatches are “single lead”, meaning they use just one electrode sensor to measure the electrical activity of the heart,’ says Ruth Goss, a cardiac nurse at the British Heart Foundation.
She adds that harmless extra heartbeats, exercise, movement or poor contact between the watch and skin can all produce unusual readings.
‘Smartwatches aren’t designed to give you a diagnosis,’ she says.
‘Most ECGs in smartwatches are “single lead”, meaning they use just one electrode sensor to measure the electrical activity of the heart,’ says Ruth Goss, a cardiac nurse at the British Heart Foundation
There is also growing evidence that repeated alerts can make some users anxious and lead to more medical appointments. In a 2024 study involving 172 people with AF, one in five of those using a smartwatch or fitness tracker reported intense fear or anxiety when it warned of a possible AF episode, reported the Journal of the American Heart Association.
It wasn’t just that people wearing the devices were supposedly ill – they’d reported no more AF symptoms than non-wearers. Yet over nine months they had 50 per cent more AF-related medical appointments, heart tests and procedures.
‘AF patients who used wearables reported more cardiac-specific anxiety than those who didn’t, with 20 per cent describing intense anxiety triggered by device alerts,’ Lindsey Rosman, an assistant professor of medicine at the University of North Carolina School of Medicine, who led the study, told Good Health.
Nearly half of wearable users checked their heart rate or performed an ECG every day, often out of habit rather than because they had symptoms.
‘That kind of routine checking tends to feed anxiety rather than settle it,’ says Dr Rosman. ‘Most people don’t have the context they need to understand and respond to these health alerts.’
She adds: ‘If you don’t know what your own normal heart rate looks like, any unusual reading can feel like a red flag. A faster heart rate after climbing stairs is nothing to worry about but, without that context, an alert can feel like an emergency.’
There are similar doubts over ‘stress’ scores offered by many smartwatches.
In a 2025 study in the Journal of Psychopathology and Clinical Science, 781 students wore a Garmin watch for three months. The device tracks changes in the time between heartbeats, which can reflect how the body responds to pressure or exertion, using them to calculate a stress score. Researchers compared the watch’s stress reading with how stressed the students actually reported feeling several times a day.
‘What users reported about their stress level and what the watch reported consistently failed to line up,’ says Bjorn Siepe, a researcher at Philipps University of Marburg, in Germany, who led the study.
‘When someone is dancing, walking fast, or doing any somewhat exhausting activity, the smartwatch might think that they are stressed.
‘Then reading that you’re stressed – even though you might not have felt that way – could induce people to worry about what they are doing wrong, even though there is no good reason for doing so.’
Ruth Goss adds: ‘While these devices can be useful for tracking physical activity and health trends, it’s crucial that people understand that they are not as reliable as the medical tests at a GP surgery or hospital, such as an ECG or a cuff-based blood pressure monitor.
‘It can be easy to become fixated on the numbers and readings. For some people, wearing a fitness tracker could be more harmful to their wellbeing than not wearing one.’
