Watch numbers are estimates
Apple Watch estimates most of what it records. Heart rate tends to be close, energy much less so, so a change over several days says more than one number.
Apple Watch reads your pulse by shining light into your wrist and measuring how much your blood absorbs, then works out its numbers from that and from motion. They're good estimates, not lab measurements.
A 2026 review of 82 studies found Apple Watch heart rate close to reference devices on average, most accurate at rest and less so in workouts with irregular movement or poor skin contact. It told sleep from wake well, but sleep stages less well, often counting deep sleep as light. Steps were moderately accurate. Energy burned was the weakest, with errors that were often large and went in no steady direction. An earlier review of 56 studies reached a similar view.
So any single reading can be off. A change that holds over several days, from the same watch on the same wrist, is much less likely to be one reading's error. That's why Pajdo compares runs of days with your usual range rather than reacting to one number.
Worth a try
Mix steady cardio with intervals
Apple says most people can improve VO2 max by increasing the intensity and frequency of cardio exercise. A meta-analysis in healthy adults aged 18 to 45 found both steady endurance training and interval training brought large gains, with intervals a little ahead.
Apple Watch estimates cardio fitness during Outdoor Walk, Outdoor Run and Hiking workouts, so those keep your readings coming.
Try it: keep your regular walks or runs, and once a week swap one for intervals, like a few minutes brisk then a few minutes easy. Give it weeks, not days.
Keep reading
Sources
- The accuracy of Apple Watch measurements: a living systematic review and meta-analysis, npj Digital Medicine (Lambe et al. 2026)
- Apple watch accuracy in monitoring health metrics: a systematic review and meta-analysis, Physiological Measurement (Choe and Kang 2025)
- Monitor your heart rate with Apple Watch, Apple Support
- Track your cardio fitness levels, Apple
- Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements (Milanović, Sporiš and Weston 2015), Sports Medicine
Checked against these sources on September 15, 2026.
This is general wellness information, not medical advice. If a number worries you, or stays away from your usual for a while, talk to a doctor.