Apple Watch Ultra 4 HRV readings differ from Health app

Hi,

Discrepancies between Heart Rate Variability (HRV) readings on an AppleWatch Ultra4 and the Apple Health app typically happen because of data source priority, metric types (SDNN vs. RMSSD), or sampling frequency.

Why Your Readings May Differ

• Data Source and Priority Order: If you use third-party fitness or recovery apps (such as Welltory, Athlytic, or Bevel) or have multiple devices writing to Apple Health, the Health app aggregates data based on a prioritized Data Sources and Access list. If your iPhone or another app/device is ranked higher than the AppleWatch Ultra4 for HRV data, the summary numbers displayed may prioritize that source.
• SDNN vs. RMSSD Metrics: Traditionally, Apple Watch records HRV as SDNN (standard deviation of beat-to-beat intervals). However, newer software updates (iOS/watchOS 27) and the Health Sensing System on the AppleWatch Ultra4 can also incorporate or display RMSSD—a different mathematical formula focused on parasympathetic (rest-and-recovery) heart rate changes. If you are looking at SDNN in one place and RMSSD in another, the numbers will not match.
• Recovery HRV vs. Background / Overall HRV: The AppleWatch Ultra 4 measures both Recovery HRV (sampled while still) and high-frequency background HRV (as often as every 5 minutes). The Health app’s summary view often blends or smooths these out differently than the live or specific metric cards (like Vitals or Recovery HRV) on the watch face.

How to Check and Fix It

1. Check Data Source Priority:

• • Open the Health app on your iPhone.
  • Tap Browse > Heart > Heart Rate Variability (HRV).
  • Scroll to the bottom and tap Data Sources & Access to ensure your AppleWatch Ultra 4 is set as the top priority source.

1. Verify the Metric Type:

• • Check whether the screen or app is displaying SDNN or RMSSD, as comparing them directly will show a mismatch.