What your HRV says, and what it doesn't

Your heart doesn't beat like a metronome. The gap between beats changes a little all the time, and heart rate variability (HRV) measures how much. Apple Watch reports it as SDNN, in milliseconds. Higher usually means your body is rested and recovering well.

HRV drops with age. In a German study of 1,906 healthy adults, 5-minute SDNN averaged about 50 ms in men aged 25 to 34 and about 30 ms at 65 to 74. Women followed almost the same curve. The spread is wide at every age, so two healthy people the same age can differ by 20 ms or more.

Low HRV also goes with heart trouble later. A meta-analysis of 8 studies with 21,988 people without known heart disease found that those with the lowest SDNN had about a 35% higher risk of a first cardiovascular event than those with the highest. These studies are observational. Low HRV may be a sign of poor health more than a cause of it.

Day to day, HRV moves a lot. Short sleep, illness, stress and hard training all lower it for a while. So does alcohol: in a study of 4,098 Finnish employees who wore heart monitors, HRV in the first hours of sleep dropped on nights they drank, even after small amounts.

That's why Ageon uses the middle value of your last 14 days, and needs at least 5 days of readings. It counts in your cardio score with the same weight as resting heart rate, and it moves your Ageon Age, because it changes so much with age. A watch takes short readings through the day, not a 5-minute test lying down, so treat the comparison with others as rough.

Your own trend is more useful than any table. Try an experiment, like two weeks without alcohol, and see what it does to your numbers.