What your smartwatch can – and can’t – tell you about your health

Smartwatches can monitor your heart rate, activity, sleep and, on some devices, heart rhythm. But how much can this data really reveal about your health?

From counting steps to monitoring your heart rate overnight, smartwatches have become increasingly sophisticated health-tracking tools. Some can record an electrocardiogram (ECG), monitor blood oxygen levels, and flag possible irregular heart rhythms.

This information can help you keep track of your health, but it can’t replace a medical assessment, says Dr Kaleemuddeen Ebrahim, a physician specialising in internal medicine at Mediclinic Constantiaberg.

“Smartwatches haven’t been validated to provide full clinical data for diagnostic purposes,” he explains. “They can be useful to give an idea that something may be wrong, but you’d still need to see a healthcare provider for a formal diagnosis.”

What can my smartwatch tell me?

What a smartwatch can do is to help you notice changes in measurements, such as your heart rate, activity levels, and sleep.

“For example, if your watch repeatedly records an unusually high or irregular pulse, it’s worth discussing with your healthcare professional. Some devices can also flag possible atrial fibrillation (AF), an irregular heart rhythm that can increase the risk of complications such as stroke.

“However, a smartwatch alert does not confirm that you have AF. A healthcare professional may need to perform further testing, such as a clinical electrocardiogram (ECG), before making a diagnosis.”

The same applies to sleep tracking. “Your watch may indicate how long or restlessly you’ve slept, and some devices can monitor snoring or changes in blood oxygen levels. These measurements can be useful for spotting patterns, but they can’t provide the same level of information as a formal sleep assessment,” Dr Ebrahim cautions.

How accurate are smartwatch readings?

The accuracy of wearable devices depends on the device, the measurement being taken, and the circumstances in which it’s used.

The BASEL Wearable Study compared five commercial wearable devices with physician-interpreted 12-lead ECGs in 201 patients. Two measures matter when evaluating how well a device detects atrial fibrillation: sensitivity, or the ability to correctly identify people who have the condition, and specificity, the ability to correctly identify those who do not. 

The study also found that many recordings were inconclusive, with rates varying by device. This is one reason wearable readings should be treated as information that may need follow-up, rather than a diagnosis.

What should I do if my watch flags an abnormal rhythm?

If your smartwatch repeatedly indicates an irregular rhythm, particularly if you’re experiencing symptoms such as palpitations, shortness of breath, or unusual tiredness, speak to a GP or specialist.

Dr Ebrahim says a healthcare professional may do a formal ECG and, depending on the results, arrange further tests. These could include a Holter ECG, which records your heart rhythm over a longer period, blood tests, or an echocardiogram.

A clinical 12-lead ECG gives more detailed information about your heart than the single-lead recordings available on some consumer devices.

It’s also important to understand what your particular device is designed to measure. Some smartwatch features can flag possible AF, but wearable devices aren’t designed to identify every heart condition.

Can my smartwatch identify if I have a sleep problem?

Sleep tracking can help you spot patterns, especially if you notice your sleep changing, or you often feel tired during the day. Some devices estimate how long and how well you sleep using measurements like movement and heart rate. But they do not measure sleep the same way a clinical sleep study would.

“A formal sleep study, known as polysomnography, uses several physiological measurements, including brain activity measured through an electroencephalogram (EEG), to assess sleep stages and other aspects of sleep,” explains Dr Ebrahim.

Research suggests that consumer sleep trackers can provide useful estimates of overall sleep, but their accuracy in identifying individual sleep stages varies between devices. Rather than relying on the sleep score alone, treat it as a prompt to pay attention to how you feel.

Why can smartwatch readings be inaccurate?

Even when your device is working properly, several factors can affect its readings.

“Exercise, especially at higher levels of pulse rate, can give inaccurate readings,” explains Dr Ebrahim. “If it’s a loose-fitting watch, there may not be sufficient skin contact, and certain parameters may not be correctly monitored.”

Cold weather can also reduce blood flow to the extremities, which may affect measurements such as oxygen saturation.

So, an unusual reading doesn’t automatically mean something is wrong.

If you receive a concerning result, check that your watch is functioning correctly and you’re wearing it according to the manufacturer's instructions. It’s also useful to consider what you were doing when the reading occurred, particularly if you had just exercised or were under physical or emotional stress.

Can too much health data make you anxious?

For some people, constantly monitoring health measurements can become counterproductive.

Your heart rate and blood pressure naturally change throughout the day depending on factors such as activity and stress. A single slightly elevated reading doesn’t necessarily indicate a health problem.

Dr Ebrahim says repeatedly checking these measurements can in fact increase anxiety. “I advise my patients that monitoring every blood pressure and every pulse rate all through the day is probably counterintuitive to general health. Your body is always changing throughout the day, and blood pressure and pulse will change depending on the activity.”

Also, worrying about readings can affect your blood pressure, creating a cycle of increased anxiety and further checking.

Instead of focusing on every individual number, look for persistent changes, and consider them alongside how you’re feeling.

Use your smartwatch as a health tool, not a diagnosis

Your smartwatch can help you become more aware of your activity, sleep, and certain health trends, and may alert you to a change that deserves further investigation. The key is understanding what the device can measure, recognising its limitations, and considering the information alongside your symptoms and other health factors.