Apple Watch Ultra 4 - Ultimate Scientific Review!

Apple Watch Ultra 4 — Review Summary

Main findings

• Workout heart rate: Generally very accurate compared with a Polar H10 chest strap. Correlations were 1.00 for spinning and 0.99 for outdoor running, cycling, and weight training.

  • Compared with devices tested by the reviewer, it outperformed 98% for spinning, 92% for cycling, 74% for running, and 98% for weight training on correlation.
  • Spinning was nearly flawless. Running and cycling were strong overall, though some sessions had brief gaps or dropouts. Weight training was more variable: the watch sometimes missed peaks or lost signal during sets.
  • Apple appears to suppress readings when it considers the signal unreliable. This avoids displaying questionable measurements, but can improve correlation scores while leaving gaps in the data.
  • The new sensor does not appear to substantially improve workout accuracy over previous Apple Watches. The reviewer suggests its main benefits may be better power efficiency and more frequent potential measurements.

• Everyday heart rate: Readings matched an ECG chest-strap reference well, with 0.99 correlation during a short comparison. However, the reviewer found the exported data unexpectedly sparse. It is unclear whether this reflects Apple Health’s export or storage behavior, or how the watch measures heart rate outside workouts.

• Overnight heart rate and HRV:

  • Nightly average heart rate was close to the reference, with a maximum deviation of about 1 bpm. Some individual readings showed unexplained spikes.
  • Heart-rate variability (HRV) performed very well across nine nights, with frequent readings and patterns that generally matched the reference. The watch tended to report slightly lower HRV.

• Sleep stages: Compared with a ZMax EEG reference across roughly six or seven nights, the watch’s stage agreement was:

  • Deep sleep: 77%
  • Light sleep: 81%
  • REM sleep: 74%

The reviewer considered this solid performance, with some nights matching especially well. The overall sleep-tracking score was 77 (“great”).

Results were essentially the same as in the reviewer’s Apple Watch Ultra 3 testing. The reviewer thinks Apple uses a similar algorithm across its watches; Apple Watch Series and SE models were also described as performing similarly.

• GPS: Repeated cycling routes were mostly consistent, as were the three running tracks. However, Apple’s smoothing and anti-aliasing can remove route detail and occasionally create unrealistic paths, such as cutting through a building or pond, or showing an impossible route across a square. The GPS score was 80 (“good,” not great). Overall performance was roughly similar to the Ultra 3, perhaps slightly worse, though the reviewer was uncertain.

Pros

• Excellent workout heart-rate accuracy when the watch has a good signal.
• Strong overnight HRV readings and good average nighttime heart-rate accuracy.
• Above-average sleep-stage tracking, particularly compared with many competitors.
• Generally consistent GPS tracks and reliable overall health tracking.

Cons and caveats

• Occasional heart-rate dropouts, especially during some runs, bike rides, and weight-training sets.
• Daily heart-rate data exports seemed much less frequent than expected; the reason was unclear.
• Unexplained nighttime heart-rate spikes appeared in some exported data.
• GPS smoothing can obscure detail and produce implausible route segments.
• Some testing used beta or developer software, which could have contributed to issues.

Comparisons and ratings

• The Polar H10 chest strap was the heart-rate reference.
• Sleep-stage performance was similar to the Apple Watch Ultra 3 and broadly comparable across Apple Watch models.
• The reviewer also cited Fitbit/Pixel devices, Whoop, and Eight Sleep among strong sleep-tracking performers in prior testing.
• Overall heart-rate tracking scored 99 based on correlation alone. The reviewer said dropouts could justify lowering the rating to “excellent” or “great,” depending on how much missing data matters.
• Overall verdict: A very solid health tracker, with excellent sleep-stage and HRV performance, strong workout heart-rate accuracy, and good—but not perfect—GPS.

Reviewer’s perspective

The review and ratings are based on Rob’s measurements against reference devices. He emphasized that results can vary between users and that he had heard mixed reports about the new sensor.