Millions start their day with a number readiness, strain, or body battery delivered by wearable devices. These composite health scores promise to simplify complex signals like sleep, heart rate, and activity into a single, actionable metric. But how reliable are they?

Experts warn these scores often rely on optical sensors and algorithms that lack transparency. Small inaccuracies like in sleep tracking or heart rate can skew results, and some devices “double-dip” by penalizing the same stressor twice (like bad sleep and its impact on heart rate variability).

Further, most wearables don't reveal how scores are calculated or personalized, and inconsistent baselines make cross-device comparisons meaningless.

Still, these trackers can be helpful when used for long-term trends, not daily decision-making. Let your body be the final judge, not a score. Your fitness tracker is a helpful guide, but it's not your doctor.