HRV Metrics: SDNN
SDNN is the Standard Deviation of the NN (another term for R-R) intervals - the intervals between heartbeats. SDNN is more accurate when calculated over 24 hours than during shorter periods. Both Sympathetic Nervous System and Parasympathetic Nervous System branches contribute to SDNN.
For healthy individuals ages 25-74 years, the range is 16-70.9. You can find your specific healthy range in the Population Comparison feature of the app!