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Science and evidence

Research informs Metric. Your own trends provide the context.

Metric uses published research and established measurement conventions to help organize wellness signals. Research can explain why a metric may matter at a population level; Metric then shows how that signal appears in a person’s own Apple Health history without turning an association into a diagnosis.

Last reviewed July 16, 2026

How evidence is used

  • Prefer primary studies, consensus papers, official Apple documentation, and public-health guidance over unsourced summaries.
  • Separate what a source reports from Metric’s interpretation and from the way the app calculates or displays a result.
  • Describe associations as associations. A change in two signals at the same time does not by itself establish cause.
  • State important measurement limits, including device fit, wear time, sampling frequency, missing data, and differences between people.
  • Use dates and direct source links so readers can inspect the evidence and see when a page was reviewed.

Core source library

Apple HealthKit documentationApple Developer DocumentationDaily steps and mortality in adultsThe Lancet Public Health, 2022Recommended adult sleep durationSleep, 2015Sleep regularity and mortality riskSleep, 2024Physical Activity Guidelines for AmericansU.S. Department of Health and Human ServicesCardiorespiratory fitness as a clinical vital signCirculation, 2016

What research cannot tell you from an app screen

Population studies can establish useful reference points, but they cannot determine why one person’s metric changed on a particular day. Consumer wearables also estimate some measurements and sample others intermittently. Metric therefore emphasizes direction, data coverage, and multiple related signals rather than treating a single reading as a conclusion.

If a value is concerning, unexpected, or connected to symptoms, discuss it with a qualified healthcare professional.