SDNN

What it is

SDNN is the standard deviation of the intervals between normal heartbeats (the “NN” intervals, with irregular beats removed). It describes the overall spread of those intervals over a recording, so it captures both fast and slower swings in heart rate.

Why it matters for training decisions

If you use an Apple Watch, SDNN is the HRV number you’ll see in Apple Health. Apple Watch measures it opportunistically from short readings taken in the background when you’re still, and during Breathe sessions in the Mindfulness app.

SDNN depends heavily on the length of the recording. A 24-hour clinical SDNN, a 5-minute SDNN and a 1-minute watch reading are not interchangeable, and published norms usually refer to a specific length. Like any HRV measure, it’s most useful compared with your own past values taken the same way.

How wearables measure it, and the limits

Apple’s tachogram algorithm records the time between beats using the optical sensor and motion data, but only when you’re still enough for a clean reading. That means the number and timing of readings change from day to day with how active you are. A day with one reading after a stressful meeting and a day with several overnight readings aren’t directly comparable.

SDNN and RMSSD answer slightly different questions, so an Apple Watch value and a WHOOP or Oura value for the same night will differ.

How P247 uses it

The HRV P247 reads from Apple Health is SDNN-based. It’s one input to the daily recovery score, where it’s rated against a fixed reference value. Separately, P247’s HRV status label compares it with your own earlier values from the same measurement source.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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