Overnight rMSSD
Sixteen uses adjacent beats only, rejects artefacts and pools the night, which is why the number is steadier than a 60 second morning test.
HRV tracking
HRV with a baseline, so the number means something.
A single HRV reading is noise. A week against your own range is a signal. Sixteen measures overnight rMSSD, sets your normal band and tells you when you drift out of it.
Your nervous system, one number a day.
HRV is the gap between beats, measured while you sleep, and it moves before you feel tired.
Your normal band
A rolling baseline with a smallest worthwhile change, so a 4 ms dip is shown as noise and a 12 ms drop is shown as a real one.
Seven day view
Average, high and low for the week with each day marked very high, high, normal or low.
Linked to recovery
HRV is the heaviest input to your recovery score, so the two pages always agree with each other.
Then it changes the day
Measured is not the same as changed.
When HRV drops, Sixteen does not just show a red bar. It lowers the day's strain target, suggests a breathe session and holds your evening app lock a little earlier.
How earning screen time worksQuestions people ask
Where do I find HRV on Apple Watch?
On the iPhone: Health, Browse, Heart, Heart Rate Variability. The watch itself does not show it. Sixteen shows the overnight value with a baseline band and a seven day chart.
Why is my Apple Watch HRV all over the place?
Apple Watch samples HRV in short windows, so single readings swing with breathing and movement. Compare the trend, not one night. Sixteen scores against a rolling baseline with a normal band for exactly this reason.
Which HRV metric does Sixteen use?
rMSSD, the root mean square of successive differences between heartbeats, measured overnight. It is the standard for short term, parasympathetic HRV and the one most wearables report.
Can the Apple Watch measure HRV?
Yes. Apple Watch writes HRV (SDNN) samples to Apple Health, and with a wear to bed habit Sixteen has enough overnight data to build a baseline. WHOOP straps provide beat to beat intervals over Bluetooth, which Sixteen turns into rMSSD itself.
What is a good HRV?
There is no universal good number. HRV varies enormously between people and falls with age. What matters is your own trend. Read our guide, What is HRV, for the detail.
Does Sixteen diagnose anything from HRV?
No. HRV is a fitness and recovery signal, not a diagnostic one. Sixteen is not a medical device.
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