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Wearables and Privacy: What Really Happens to Your Health Data When Pacing

Wearables offer useful signals for pacing – but how accurate are they really, and who sees your health data? A sober look at tech and privacy.

Many people with ME/CFS, long Covid or POTS wear a fitness tracker or smartwatch to keep an eye on resting heart rate, heart rate variability (HRV) or sleep. This can be a helpful additional piece of information for pacing. Two questions that come up less often, but matter just as much, are these: how technically reliable are the numbers your device actually shows? And what really happens to the health data generated in the process? This article deliberately does not revisit the interpretation of HRV or resting heart rate for pacing, but instead looks at the technical reliability behind the numbers and how the resulting data is handled.

How accurate are consumer wearables really?

Most wrist-worn wearables use a technique called photoplethysmography (PPG): a light source measures changes in blood volume through the skin, from which heart rate and HRV are calculated. This method is convenient, but sensitive to movement, skin contact and ambient light.

A recent study tested five common wearables (Garmin, Oura Gen 3 and Gen 4, Polar, Whoop) against an ECG as reference over 536 nights. Interdevice accuracy varied significantly (p < 0.05). For resting heart rate, Oura Gen 3 and Gen 4 achieved the highest accuracy, while Polar showed weaker and WHOOP moderate agreement. There were also notable differences between devices for HRV. The authors also point to a fundamental problem: each device uses proprietary algorithms, and there is little transparency about which measurements feed into the respective "readiness" or "recovery" score. These algorithms are also regularly updated, which means calculated values can change over time even though nothing has changed in the body.

Another study looking at four PPG-based heart rate wearables reaches a similar conclusion: PPG-based wearables raise critical questions about validity. A 24-hour real-world study with the Apple Watch and Fitbit also showed that devices can be fairly accurate at rest, but that deviations vary under everyday conditions – and these studies were mostly carried out on small groups of healthy, often young adults, not on people with dysautonomia or chronic fatigue. This doesn't mean the numbers are useless – but they are a rough guide, not a medical measuring instrument. Individual outliers or an unusual reading on a single day shouldn't be over-interpreted; trends over several days or weeks are more meaningful.

Where does your data actually go?

Health data from wearables is classified as particularly sensitive under the law. Under the European General Data Protection Regulation (GDPR), health and fitness data from smartwatches, fitness trackers and health monitors are considered sensitive health data under Article 9, which requires the highest level of protection and explicit consent for processing. In practice, this means manufacturers must clearly justify what they use pulse, sleep or activity data for, and you have to actively consent to that use.

Even so, analyses of actual practice show that many users have little overview of where their data really ends up – for example, on manufacturers' cloud servers, in linked third-party apps, or with advertising partners. For instance, Fitbit was accused in a 2011 class-action lawsuit of selling personal health data to third-party advertisers without users' consent – an example of how privacy promises and actual practice can diverge. It's worth knowing that even if a device or app "only" counts sleep or steps, this data, combined with other information, can allow conclusions to be drawn about your state of health – for example, about periods of exhaustion or symptom flares.

What you can specifically check

  • Does the privacy policy clearly state which data is collected, stored and, if applicable, shared?
  • Is there a way to view, export or have stored data completely deleted?
  • Can cloud syncing, third-party analytics or ad tracking be individually switched off in the settings?
  • Is data encrypted during transmission and storage?
  • If the device is provided through an employer or an insurance programme: who has access to your readings there?

Using wearables sensibly, without blind trust

In everyday pacing terms, this means: a wearable can provide an additional, supportive signal – something worth following up on, not proof and not a diagnosis. Fluctuations in resting heart rate or HRV have many possible causes, ranging from sleep quality and caffeine to measurement errors in the device itself. If readings differ strongly from how you actually feel, that's no cause for concern – it's simply a reminder that technology has its limits.

It's equally worthwhile spending a few minutes on the privacy settings when choosing and setting up a device – this not only protects your privacy but also helps avoid unnecessary data sharing, especially for a topic like chronic illness that many people would rather keep control of themselves.

mypacing is not a medical device and does not replace medical diagnosis or treatment. If you have questions about heart rate, circulatory symptoms or a possible POTS work-up, please consult a medical professional – wearable data can, at most, be an additional talking point, not a basis for self-diagnosis.

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