What is "Undefined"?

Generally, higher HRV indicates lower stress and a better-balanced nervous system. However, when HRV spikes dramatically above your normal range, StressMonitor marks the reading as Undefined rather than showing a stress level.

Here are the most common reasons for an abnormally high HRV reading:

1. Potential Heart Issues

If your HRV (RMSSD) consistently exceeds 300ms, this is not a positive sign. It may indicate abnormal heart function such as premature beats. Consult a doctor for a professional medical evaluation.

2. Naturally Slow Heart Rate

If your heart rate consistently remains below 50 bpm — common among athletes or individuals with naturally low resting heart rates — your RMSSD readings may frequently appear elevated (often above 200ms). This is mathematically expected given the longer intervals between beats.

Even in this case, consulting a doctor and following their advice is recommended.

3. Overexertion

Athletes in endurance sports sometimes experience an HRV spike during periods of intense training. Rather than indicating recovery, a sudden HRV rise in this context may reflect the parasympathetic nervous system stepping in for emergency recovery — a sign the body is overworked, not improving.

If you notice unusually high HRV readings for one or several nights, consider stopping your current training load and resting.

4. Measurement Error

Inconsistencies in how the Apple Watch is worn can produce a single abnormally high reading due to sensor noise. If this appears only once and your readings return to normal, it can generally be disregarded.

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The neural activity behind excessively high HRV values is complex and not yet fully understood by the scientific community. StressMonitor marks these readings as Undefined to avoid misleading conclusions. Interpret your readings in the context of your own situation.