Your tracker sees the surface.
We look at the science underneath.

What the trackers genuinely get right

There is a lot to benefit from trackers, and the strongest evidence is where we expect, step counting. When researchers pooled 39 reviews covering more than 160 000 people, wearing an activity tracker they added roughly 1800 steps a day and about 40 extra minutes of walking, with some modest weight loss to go with it. Resting heart rate comes out accurate to within a few percent and the newer watches are genuinely good at catching an irregular rhythm like atrial fibrillation, the kind of the thing that can get someone to the cardiologist before a stroke does.

Where the numbers turn into guesses

The trouble starts when the lower evidence-based scores get the same trust as the higher ones. Sleep is where you will see this occur more commonly. A ring or watch can tell reliability whether you are asleep or awake. What it does not do reliably is tell you how much deep sleep or REM you got and it starts guessing. These devices tend to overestimate how long and how well you slept, and they get the stages wrong often enough that the World Sleep Society tells people to use them for the basics only. The readiness and stress scores are even more shakier, a private formula turns your data into a number that looks precise, and when researchers went to outlook across roughly 310 consumer devices, only about one in ten had been validated for even a single measurement.

The weak spots of these devices are seen in both men and women. Wrist heart rate numbers hold up at rest and during easy movement, then get into training hard, this is where accuracy can fall by about a third. That is why a serious lifter, or runner is better off with a chest strap for real workout numbers.

A person’s skin tone matters too. The green light these devices shine into your skin is absorbed differently by more melanin skin individuals, which can weaken the signal and has been linked to less accurate oxygen readings, with real clinical consequences, and it affects men and women alike. Then there is the trap that catches the most people, the false sense of certainty. A reassuring green morning can talk someone out of taking a real symptom seriously, and a bad score can send a perfectly healthy person into worry and a round of tests they never needed.

The part almost no one is told: most of this was tested on men

When scientists reviewed the studies behind these products, the people used to prove the devices worked were men and only about 42 percent women. One large analysis looked specifically at how accuracy shifted with the sex of the people studied, and the error on both heart rate and calorie estimates ran significantly higher in the studies that included more women. These devices were made to male physiology, not females.

A woman’s heart rate variability, resting heart rate, and temperature move in a rhythm across the menstrual cycle. Variability runs higher in the first half of the cycle and falls in the second. Resting heart rate climbs in the days before a period. If a woman is taking birth control it shifts the whole picture, and in menopause the variability tends to settle lower across the board. Your device has no idea how to connect it to what is actually happening with the ebbs and flows of hormones at the different stages of a woman’s life. So the women whose recovery score sinks the week before her period is not broken.

Men are not exempt from being misread, its misread differently. Testosterone runs on its own daily rhythm and drifts down with age, and a man’s resting heart rate and recovery swing with a short night or a hard workout the same way anyone does. The device flattens all of it into one tidy score against a generic baseline and many interpret it as the whole story.

The one thing they did study well in women

There is a place the technology has actually done its homework and its the temperature-based cycle and ovulation tracking on two specific devices, which did hold up in real validation studies, and it does a decent job estimating ovulation and predicting when a period will start. The irony is that the female features that got research attention are the reproductive ones. The everyday numbers, the heart rate and the sleep that anyone checks are still being scored against a baseline built for someone else. None of this means take the ring off. Plenty of us wear one. These devices are useful once you know what they can and can’t tell you.

If your numbers and your body have been telling you different stories, that is worth a conversation with your preventive doctor. If you are not yet with us, book a complimentary discovery call, and we can help you understand your own data, looking beyond baseline and normal labs, we treat the person not the numbers.