Millions of people now start their day with a number 鈥 a score, a 鈥 level or a measure of 鈥 鈥 delivered by the wearable device on their wrist or finger. But how much trust should we place in these scores?
Composite health scores are increasingly used by digital fitness trackers to offer a single, daily number that reflects how your body is coping with recent demands. Whether it鈥檚 marketed as a measure of energy, recovery or resilience, the idea is the same: combine several internal signals into one clear indicator of how prepared you are to take on the day.
The concept has clear appeal. It simplifies complex physiological data 鈥 things like heart rate, sleep and activity 鈥 into an actionable recommendation: push harder, take it easy, rest. But how solid is the science behind these scores? My colleagues and I recently conducted a of the most widely used composite health scores in wearable devices to find out.
First, what goes into these scores? Typically, quite a lot 鈥 at least on paper.
Most composite health scores pull data from several biometric signals 鈥 measurements from your body that indicate how it鈥檚 functioning. These include resting heart rate, heart rate variability (the variation in time between heartbeats), sleep quantity and quality, recent physical activity, and sometimes breathing rate, skin temperature and blood oxygen levels.
On paper, that鈥檚 a rich dataset. These signals reflect how your body responds to stress, recovers overnight and balances exertion with rest. But while the inputs may be rooted in physiology, the final score can be less informative than it appears.
One issue is . These devices rely on optical sensors and motion tracking to estimate what鈥檚 going on inside your body, such as your sleep stages or daily stress levels.
Even small inaccuracies in measuring heart rate or movement can distort the score. And since these metrics feed directly into the algorithm that calculates your "readiness鈥 or 鈥渟train鈥, small errors can add up.
Another challenge is . Most companies don鈥檛 disclose how exactly they turn raw data into a final score.
We don鈥檛 know which inputs matter most, how they鈥檙e combined or whether they鈥檙e adjusted for individual differences such as age or fitness level. Without that clarity, it鈥檚 difficult to evaluate how meaningful or personalised the number really is.
A more subtle issue lies in the way certain physiological signals overlap. For instance, poor sleep is often followed by lower heart rate variability 鈥 a common sign of stress or incomplete recovery. But many health scores penalise you for both factors separately: once for the bad sleep and again for the resulting change in heart rate variability.
This kind of double-dipping can exaggerate the effect of a single 鈥渟tressor鈥 (things that put pressure on your body or mind), making your body seem more run down than it truly is. It creates the illusion of a sophisticated analysis, but may actually be highlighting the same signal twice.
Similarly, some scores penalise you for the activity you did yesterday, regardless of how well you鈥檝e recovered from it. If your heart rate variability and resting heart rate suggest you鈥檝e bounced back, that should be reflected in your score. But some algorithms still factor in recent exertion as a negative, even when your body is clearly coping well.
To make these scores more personalised, many devices compare your daily data to your typical values 鈥 your baseline. If your sleep or recovery looks significantly different from your recent average, the score adjusts accordingly.
That鈥檚 a sensible idea in theory. But there鈥檚 no standard for how these baselines are calculated. Some devices use seven days of data, others 28. Some exclude outliers; others include them. Each company defines it differently, which makes comparisons between devices impossible and raises questions about consistency.
Should you stop using your wearable?
Not at all. Fitness trackers can still offer valuable insights. Watching how your core physiological signals shift over time 鈥 from week to week or season to season 鈥 can help you spot patterns, improve habits and better understand your body鈥檚 response to stress and training.
The problem is when we treat the daily score as a definitive measure of health. It鈥檚 not a diagnosis, and it doesn鈥檛 always reflect what鈥檚 really happening inside your body. So while it鈥檚 fine to glance at your readiness or recovery score, don鈥檛 let it dictate your decisions.
Use your fitness tracker as a guide, but not as your coach, your doctor, or your judge.
By , Assistant Professor in the School of Public Health, Physiotherapy and Sports Science,
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