Putting Wearables in Perspective for Pacing: How Accurate Are Resting Heart Rate, HRV and Sleep Data – and Who Sees Your Data?

Many people living with ME/CFS, Long COVID or POTS use smartwatches or rings to keep an eye on resting heart rate, heart rate variability (HRV) and sleep. These devices can provide helpful trends – but they are not medical measuring instruments. This article takes a closer look: how reliable are the numbers your wearable shows you, really? And what actually happens to the sensitive health data generated along the way?

How accurately do consumer wearables really measure resting heart rate and HRV?

Wrist-worn wearables and rings typically measure pulse optically via photoplethysmography (PPG) sensors – detecting light reflection in tissue – rather than via an electrocardiogram (ECG), the medical gold standard. A recent validation study tested five popular devices against an ECG reference system over 536 nights. The results reveal clear differences between manufacturers: for resting heart rate, some devices showed the highest accuracy, while others achieved only moderate to poor agreement with the ECG. The differences were even larger for HRV: here too, some devices delivered the highest accuracy, while others showed only poor agreement, making HRV readings overall more error-prone than plain pulse values.

An additional problem is that the exact calculation methods often remain a trade secret. Each device uses proprietary algorithms, and while some manufacturers offer some insight into how they are designed, there is little transparency into what metrics actually feed into "readiness" or "recovery" scores. On top of that, these algorithms can be updated periodically, altering how resting heart rate or HRV are calculated – so a sudden jump in your numbers doesn't necessarily reflect a change in your body.

Movement, posture and individual differences also play a major role. A study conducted under everyday conditions found the highest agreement between devices during rest, while HRV metrics, especially in the frequency domain, showed greater error and context sensitivity, and device agreement varied notably between individuals. In practice, this means a single HRV reading on a single morning tells you very little. Trends observed over several days or weeks, measured with the same device under comparable conditions, are far more informative.

Sleep data: a useful clue, not a diagnosis

The picture is similar for sleep tracking. Wearables are fairly reliable at detecting whether you are awake or asleep. Distinguishing between individual sleep stages – light, deep and REM sleep – is considerably harder. A systematic review comparing several popular devices against polysomnography (the clinical gold standard, which includes brain-wave, eye-movement and muscle-activity measurements) found that some devices showed the smallest deviation for REM sleep and higher sensitivity for deep and REM sleep than others tested, though all devices still had clear room for improvement. Another study using simultaneous polysomnography found a similar pattern: for distinguishing sleep from wakefulness, sensitivity exceeded 95 percent for all devices, while for distinguishing individual sleep stages, sensitivity ranged from only 50 to 86 percent depending on the device.

In other words: wearable data can show whether your overall sleep time is shrinking or you're sleeping more restlessly than usual – a potential clue that, combined with your symptom diary, may be worth noting. But they are not suitable for determining whether a sleep disorder such as sleep apnea is present; that can only be established through medical evaluation, potentially including a lab-based sleep study.

What this means for pacing in practice

• Trends, not single readings: Watch for changes over several days rather than the value from a single morning.
• Stick with one device consistently: Because different devices – and even different software versions – calculate values differently, readings from different wearables aren't directly comparable.
• Your body awareness still matters most: Wearable data can be an additional data point, but it doesn't replace your own perception of fatigue, exertion limits, or the onset of post-exertional malaise (PEM).
• No self-diagnosis: Unusual patterns in heart rate, HRV or sleep are not a substitute for medical evaluation – especially with POTS-typical symptoms or newly occurring complaints, these should be discussed with a healthcare professional.

Data privacy: where does your health data actually go?

Resting heart rate, HRV and sleep patterns are not trivial fitness metrics – legally, they belong to a category of data requiring special protection. Under the EU's General Data Protection Regulation (GDPR), personal data collected through wearable devices that concerns an individual's health is categorised as special category personal data, which has a higher level of legal protection and requires one of the special conditions of processing, such as the data subject's explicit consent. A simple checkbox buried in general terms and conditions is not legally sufficient for this: valid consent must be freely given, specific, informed, unambiguous, documented, and withdrawable.

In practice, this means as a user you should consider:

• Check the privacy policy: Does it clearly explain which health data is collected and why – only for the product itself, or also for advertising, research, or sharing with third parties?
• Server location and sharing: Providers must ensure that any third parties processing the data also comply with data protection requirements and notify them promptly in case of a breach.
• Consent that's truly optional: Can you decline specific data-sharing options (e.g. sharing with research projects or third-party cloud services) individually, without losing the device's core functionality?
• Export and deletion: Check whether you can export your data and fully delete your account, including all stored values.

These questions matter especially if you collect health data over long periods to spot patterns for pacing purposes – because this kind of long-term data is potentially of interest to third parties (such as insurers or advertisers), even though reputable providers rule this out.

Conclusion

Wearables can be a useful, complementary tool for pacing, helping to make trends in resting heart rate, HRV and sleep visible. But they are not medical measuring devices, their accuracy varies considerably depending on manufacturer and situation, and the data they collect is legally classified as particularly sensitive. Use the numbers as just one building block among several – alongside your body awareness, a symptom diary, and the assessment of your treating healthcare professionals. mypacing.app is a tool to support self-observation and does not replace a medical diagnosis or a doctor's treatment recommendation.