It’s Christmas Eve 2023, and I’m in week three of a self-proclaimed sickness pit. After contracting Covid at the start of December, I’ve been blessed with an Advent featuring flu, fever and infection. That I’ve also been trialling Whoop’s latest fitness tracker means I have the data to prove it. Alongside my resting heart rate, sleep performance and respiratory rate, Whoop records my heart rate variability (HRV) – the shift in timing between my heartbeats.

A high score of the latter indicates your body is able to adapt to stressful situations, while a low score suggests the opposite. Since mine plummeted 40 milliseconds from my average, I’ve been checking it daily – and having access to these stats means that on the odd day it increases (fluctuations are normal), I’m reassured I can manage a 10-minute stroll without exacerbating my symptoms – or burdening my local GP surgery at its busiest time of year.

How could fitness watches detect sickness?

I’m sure the good people at Whoop will be pleased to hear it; the ability to detect and monitor illness is a core component of the brand’s mission to ‘unlock human potential’. And they’re not the only wearable with this goal. Garmin, Fitbit and Oura use similar metrics to curate ‘stress’ graphs and ‘daily readiness’ scores, with ratings intended to inform how active a user is that day.

Huawei’s WatchD is the first and only wearable with the ability to take a 24-hour blood pressure measurement ‘six times more effective at preventing mortality than in-clinic blood pressure measuring’, while the first dedicated illness tracker, Visible – designed using a Polar heart rate monitor, with heart rate (HR) and HRV metrics used as a proxy for sickness – launched earlier this year.

If the technology is new, the sentiment isn’t. Apple launched an electrocardiogram (ECG) app back in 2018, to identify irregular heartbeat patterns and uncover heart-related conditions via a 30-second reading, with the added option to schedule periodic scans that alert you to abnormalities. The tech giant is now planning to launch a ‘sick mode’ at the end of 2025, where users can pause their activity rings if they need to rest – meaning they won’t lose that all-important streak. So, with so many brands working to encompass personal health among its tracking capabilities, can you really use your fitness watch to track sickness? The answer from the women in my DMs is an emphatic ‘yes’.

Real-life stories: the women who have detected sickness using their fitness watches

When I put out a call on my socials, I was inundated with stories of illness ID’d by the heart rate data served up by the tracker on their wrist. Cass Davis tells me her Fitbit helped her identify tachycardia – a condition that makes your heart beat faster than normal – after being put on an old tricyclic antidepressant, amitriptyline, which she was prescribed at a low dose to help nerve pain. ‘Over the two months I started medication, I noticed my resting heart rate increasing – it was 61bpm at the start of May and 71bpm by the end of July,’ shares the 34-year-old.

At first, she blamed her coffee habit, but when it increased throughout August, she booked to see her GP. ‘They said it was a side effect of my medication, so I was taken off it straightaway and given an ECG to check for heart damage. Thankfully, there wasn’t any, but without the heart rate data on my watch, I wouldn’t have gone to the doctor as quickly and my condition could have become critical.’

It’s a similar story for Karen Strong. ‘I noticed my heart rate would randomly increase by about 10bpm while I was resting on the sofa,’ says the 57-year-old, on the change that first aroused her suspicions that she was in menopause. (Experts say menopausal heart palpitations can increase heart rate by 8 to 16 beats.) ‘This continued sporadically, two or three times per week for about a month, before I booked a GP appointment.’ Subsequent tests connected this symptom with lower oestrogen levels and – given Karen had also started to experience hot flushes – she was put on HRT to help.

So, how reliable are fitness watches for detecting sickness?

That your fitness tracker may know your health status before you is supported by science. A study published in PLOS Biology used fitness watches to measure participants’ baseline HR, blood oxygenation and skin temperature. When deviations from these baselines coincided with times participants became ill, the authors concluded that wearables can be used to signal the onset of both common colds and conditions as varied as type 2 diabetes and Lyme disease. In September this year, another review on 1.2 million participants found fitness watches could correctly identify Covid in 88 out of 100 people and atrial fibrillation – a condition that causes an irregular and fast heartbeat – in 87 out of 100. It isn’t the first study to look at this phenomenon in the context of Covid, either.

Back in 2020, researchers at the University of California found that the skin temperature measured on an Oura ring could detect asymptomatic Covid – participants with Covid recorded a higher skin temperature – while in the same year, other research found that respiratory rate, HR and HRV measurements using a Fitbit helped to predict the onset of Covid.

The first corroborative investigation was led by Michael Snyder, director at the Stanford Center for Genomics and Personalised Medicine.‘[Smartwatches] can detect respiratory illnesses such as tuberculosis and bronchitis in advance of symptoms 80% of the time,’ he shares, on why he’s a strong proponent of fitness trackers as sickness trackers. But when I ask him how reliable he thinks they are, there’s a ‘but’.

‘The biggest issue is that metric abnormalities can be triggered by stress. For example, your resting heart rate (RHR) may increase, and your HRV may decrease, but this doesn’t always mean you’re physically unwell.’ Then there’s the data-warp that being a woman delivers. ‘RHR and skin temperature can vary according to different phases in the menstrual cycle,’ he shares, adding that RHR should only decrease or increase by two to three bpm during your cycle. ‘As for the perimenopause and menopause, a reduction in oestrogen and progesterone can mean a higher heart rate and lower HRV.’

When I reach out to the British Heart Foundation, its senior cardiac nurse Chloe MacArthur was cautiously optimistic. ‘Devices that are strapped to your chest to monitor your heart’s electrical activity are generally more accurate than devices worn on your wrist or smartphone apps, which mostly work by using light to measure changes in blood flow under your skin,’ she tells me. ‘Heart monitors worn on your wrists may produce an inaccurate reading if you move your arm too vigorously.’ She also underscored, in no uncertain terms, the importance of seeking medical attention if you have sudden or persistent chest pain that feels like pressure, tightness or squeezing.

To be clear, no one is suggesting a watch should replace a doctor. ‘Watches are useful for telling if something is off, but not for clinical-grade diagnoses,’ adds Dr Snyder. ‘Think of any unusual metrics as an engine light in your car; the car tells you if something is wrong, but you need a mechanic to tell you what the problem is.’ And while some cardiologists and medical specialists have expressed concern that smartwatches could raise false alarms – meaning unnecessary follow-up tests and healthcare costs – most feel the positives outweigh the negatives. For Dr Snyder and MacArthur, the ideal is that patients can respond to health concerns quicker and avoid serious consequences. Watches help predict illness, doctors diagnose it.

As I write, this topic feels pertinent. The PM recently outlined his 10-year plan to transition the NHS into a ‘neighbourhood health service’, prioritising digital consultations and moving care from hospitals to communities, after the NHS was deemed to be in a ‘critical condition’. Sure enough, data has shown that chronic NHS backlogs last year led to a 7% increase in private admissions from 2022 to 2023.

But not everyone can afford that, and the same goes for wearable health devices. You’d be hard pushed to find a watch for less than £100, and even if you are able to invest in one, access to a free health service like the NHS is a privilege, and one I certainly don’t take for granted. While I relied on my Whoop for guidance on whether I could manage a daily walk when battling through Covid, it was my GP who prescribed me antibiotics to fight off my infections; my GP who gave me an inhaler to help me breathe; my GP who, eventually, helped me get better.

Should anyone avoid using fitness watches to track sickness?

Of course, sickness tracking won’t work for everyone – not least the 10% of the population believed to have health anxiety, because studies show that ambiguous data, such as transient exercise-induced HR elevations, is often misinterpreted as a health threat. It’s recommended that you wear your watch 24/7 for the most accurate data, with the most reliable numbers taken during sleep, given wake-time activities can alter and elevate your heart rate.

‘Having access to these kinds of metrics should help you feel more reassured in your body because you know what’s going on and can treat potential issues sooner,’ says Dr Snyder, when I ask his advice for safeguarding your mental health. ‘But I’d recommend turning any alerts off, and only checking your metrics once a day, or if you feel unwell.’

And yet, if the sickness trackers of today require a careful approach to mental-health management, the sickness trackers of the future could work with your mind. ‘I believe people will eventually be able to diagnose without a doctor’s appointment,’ adds Dr Snyder, when I ask where the technology is heading. ‘Watches will alert you
if something is wrong, then take you to a dashboard with a virtual explanation of your diagnosis from a doctor and how to treat or manage symptoms. This will apply to both mental and physical health – you’ll be able to identify mental disorders, such as anxiety, too.’

Oh, and if you’ve only just got on board with ‘wearables’, get ready for ‘implantables’. ‘Instead of wearing something around your wrist, you would inject a wireless chip,’ he adds, in a prediction he believes will play out within the next decade. His forecast isn’t too far-fetched; in 2021, engineers at Columbia University School of Engineering and Applied Science developed exactly that – an ultrasound-powered, injectable, fully functioning single-chip system that’s so tiny it enters the body by way of a hypodermic needle. The chips have yet to be injected into human subjects, but trials are in the pipeline. And, you know what? If it means I don’t miss Christmas 2034, I’m in.

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Which illnesses can fitness trackers reliably detect without symptoms?

‘The below conditions are all detectable via fitness trackers,’ says Dr Snyder. ‘But, while they may identify the illness before you see a medical professional, your watch can’t diagnose you.’ Always visit your GP if your metrics fit the criteria in the section below.

• The common cold
• Bronchitis
• Laryngitis
• Tuberculosis
• Covid-19
• Lyme disease
• Atrial fibrillation
• Stress
• Tachycardia

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Some of my metrics have moved from my baseline. When should I see a doctor?

1. Resting heart rate (RHR)

• Difference from average: 10 to 20 beats per minute higher
• When to book: If these numbers are consistent over a week

2. Blood oxygen

• Difference from average: Below 90%
• When to book: Book a same-day appointment – don’t panic, but it’s worth checking out

3. ECG reading

• Difference from average: Any ‘abnormal’ ratings
• When to book: If two separate ECG readings comeback as abnormal

4. Heart rate variability (HRV)

• Difference from average: 10ms or more below your average
• When to book: If these numbers are consistent over a week

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Read now: How to build compassion and a stable sense of confidence

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