What is HRV, and why a healthy heart doesn't beat evenly

Take your pulse and count carefully, and you'll find your heart doesn't beat evenly like the hand of a clock. The gap between one beat and the next is a little longer, then a little shorter. That difference is called HRV (heart rate variability), and it is one of the numbers many wearables report every morning.

What HRV is

HRV is the variation in time between consecutive heartbeats, measured in milliseconds (ms). A review by Shaffer and Ginsberg (2017) puts it simply: "a healthy heart is not a metronome." A rhythm that keeps shifting is a sign that the body is adjusting to what is around it.

There are several ways to measure HRV. A common one is RMSSD, which the same review describes as the main time-domain measure used to estimate vagal activity, the part of your nervous system that works when your body gets to rest.

Is higher always better

Not always. Shaffer and Ginsberg note that higher HRV is not always better, because some abnormal conditions can also raise it. HRV also varies a lot from person to person, and time-domain measures tend to fall with age. Comparing your number with someone else's tells you very little.

What lowers HRV in a single night

One example with clear data is alcohol. A study of 4,098 Finnish employees (Pietilä et al., JMIR Mental Health 2018) compared nights with and without drinking for the same person, and found that RMSSD in the first three hours of sleep went down.

The researchers concluded that alcohol suppresses the rest side of the nervous system in a dose-dependent way. The more people drank, the larger the drop.

How to read your own number

1. Watch your own trend over several days. A single day can swing a lot. Your own average over several weeks means more. 2. On a day that runs much lower than usual, ask what happened yesterday. Did you drink, sleep too little, or start feeling unwell? 3. Don't compare yourself with anyone. Age, sex and measurement method all differ, so the number is hard to compare between people.

The Health tab in Align shows your HRV when you connect a supported wearable, so you can see the daily trend alongside your sleep and resting heart rate. You can read more about resting heart rate in the previous article.

---

*This article is general information, not medical advice. If you have unusual symptoms or concerns about your heart, please talk to a doctor.*

Sources

• Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health 2017;5:258. https://pmc.ncbi.nlm.nih.gov/articles/PMC5624990/
• Pietilä J, et al. Acute Effect of Alcohol Intake on Cardiovascular Autonomic Regulation During the First Hours of Sleep in a Large Real-World Sample of Finnish Employees. JMIR Mental Health 2018;5(1):e23. https://pmc.ncbi.nlm.nih.gov/articles/PMC5878366/