Why Are HRV Values Different from Apple Health?

You may notice that the HRV number shown in StressMonitor differs from what you see in the Apple Health app. This is expected — the two apps use different HRV calculation methods.

Apple Health Uses SDNN

Apple Health displays SDNN (Standard Deviation of Normal-to-Normal intervals) — the standard deviation of all normal heart rate intervals. SDNN reflects overall heart rate variability, capturing effects from both the sympathetic and parasympathetic nervous systems. Larger SDNN values generally suggest higher variability and stronger cardiac adaptability.

StressMonitor Uses RMSSD

StressMonitor uses RMSSD (Root Mean Square of Successive Differences) — the square root of the average of squared differences between consecutive heartbeat intervals. RMSSD primarily measures parasympathetic nervous system activity and short-term recovery capacity.

Key Differences

	SDNN	RMSSD
Scope	Overall HRV (long + short-term)	Short-term HRV only
Focus	All interval variations	Adjacent interval differences
Sensitivity	Longer timeframe changes	Short-term subtle changes
Best for	Overall cardiac autonomic assessment	Short-term stress status

Why StressMonitor Chose RMSSD

RMSSD is more sensitive than SDNN and better suited to capturing subtle, real-time changes in the body — making it a more appropriate indicator for short-term stress monitoring and alerts.

This is why the numbers look different. Both metrics are valid; they simply measure different aspects of heart rate variability.