Heart rate variability (HRV)

Your heart doesn't beat like a metronome. The gap between two beats changes slightly from one beat to the next. Heart rate variability, or HRV, measures how much. It's shown in milliseconds (ms).

Why it matters

HRV follows your nervous system. When you're rested, it tends to sit higher. A hard session, a short night, alcohol, illness or stress usually pull it down for a while. That's why Soma's Recovery and Stress scores lean on it.

Where Soma gets it

From Apple Health. Apple Watch measures HRV a few times a day and overnight. Some rings and scales write it too. Different devices measure it differently: Apple uses a measure called SDNN, and some brands use RMSSD and keep it in their own app. Compare yourself with yourself, on the same device.

How to read it

• Your own usual is what counts. HRV varies a lot between people, and with age. A number that's low for one person is normal for another.
• Look at the trend. One low morning means little. Several days below your usual is worth noticing.
• Measure the same way. Overnight readings are the most comparable from day to day.

On Today, What moved shows when your HRV sits away from your usual, and Ask Soma can explain today's number. See Scores and what moves them.

If HRV is a dash after nights with your watch, see HRV or heart recovery is empty.