Millions of people now start their day with a number — a “readiness” score, a “body battery”“ level or a measure of “strain”“ — 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’s 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 — things like heart rate, sleep and activity — 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 systematic review of the most widely used composite health scores in wearable devices to find out.
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First, what goes into these scores? Typically, quite a lot – at least on paper.
Most composite health scores pull data from several biometric signals — measurements from your body that indicate how it’s 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’s 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 sensor accuracy. These devices rely on optical sensors and motion tracking to estimate what’s going on inside your body, such as your sleep stages or daily stress levels.
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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” or “strain”, small errors can add up.
Another challenge is transparency. Most companies don’t disclose how exactly they turn raw data into a final score.
We don’t know which inputs matter most, how they’re combined or whether they’re adjusted for individual differences such as age or fitness level. Without that clarity, it’s 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 — 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.
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This kind of double-dipping can exaggerate the effect of a single “stressor” (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’ve recovered from it. If your heart rate variability and resting heart rate suggest you’ve 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 — your baseline. If your sleep or recovery looks significantly different from your recent average, the score adjusts accordingly.
That’s a sensible idea in theory. But there’s 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.
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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 — from week to week or season to season — can help you spot patterns, improve habits and better understand your body’s response to stress and training.
The problem is when we treat the daily score as a definitive measure of health. It’s not a diagnosis, and it doesn’t always reflect what’s really happening inside your body. So while it’s fine to glance at your readiness or recovery score, don’t let it dictate your decisions.
Use your fitness tracker as a guide, but not as your coach, your doctor, or your judge.
