Your smartwatch's HRV number: what it means and what it doesn't

Happy Thanksgiving week from all of us at Monarch Bay Chiropractic.

If you wear a smartwatch, fitness band or ring, there's a good chance it reports a number called HRV. This Tech Tuesday explains what it measures, why one day's number isn't a grade, and what tends to nudge it up or down.

What HRV measures

HRV stands for heart rate variability: the variation in time between one heartbeat and the next (Shaffer & Ginsberg, 2017). Even at rest, the gaps between beats aren't perfectly even. One research team put it in a paper title: a healthy heart is not a metronome (Shaffer, McCraty & Zerr, 2014).

Researchers describe HRV as the product of several body systems working together to adapt to physical and emotional challenges (McCraty & Shaffer, 2015).

How your watch gets the number

Most wrist-worn trackers read your pulse through the skin. Cleveland Clinic notes that wrist devices usually aren't as accurate for HRV as an EKG or a chest-strap monitor. A small 2021 study that compared several consumer devices with a medical-grade ECG found that accuracy varied widely from device to device (Stone et al., 2021).

Higher isn't a score to chase

• Researchers talk about an optimal level of HRV, not a maximum (Shaffer & Ginsberg, 2017). Some irregular heart rhythms can even push the number up.
• What's normal for one person may not be normal for someone else (Cleveland Clinic), and HRV tends to decline with age (Shaffer & Ginsberg, 2017).
• So compare yourself with yourself, not with a friend or an online chart.

Trends beat a single day

A short reading reflects the conditions of that moment. Shaffer and Ginsberg note that short readings aren't interchangeable with 24-hour recordings. Sports scientists who track HRV in athletes recommend averaging readings rather than reacting to a single morning (Plews et al., 2013).

The practical version: look at your weekly average and which way it's heading.

What can move it

• Sleep. Shorter or lower-quality sleep is linked with lower HRV (Cleveland Clinic).
• Alcohol. In a 2018 observational study of about 4,000 Finnish employees, evening drinking was linked with lower HRV-based recovery in the first hours of sleep. More drinks meant a bigger effect (Pietilä et al., 2018).
• Stress. Stress is associated with lower HRV (Shaffer & Ginsberg, 2017).
• Getting sick. In wearable data from people with COVID-19, HRV tended to dip during the illness (Natarajan et al., 2020).
• Training load. In athletes, HRV shifts as training gets harder or easier (Plews et al., 2013).
• Breathing. Slower, deeper breathing increases the natural rise and fall of heart rate with each breath (Shaffer & Ginsberg, 2017).

What it can't tell you

A wearable's HRV reading isn't a diagnosis. It can't tell whether anything is wrong with the heart. Anyone with symptoms that worry them should talk with their doctor, whatever the watch says.

Coming up next week on Feel-Good Friday

Breathing is the item on that list you can practice anywhere. Next week on Feel-Good Friday, we'll walk through heart-focused breathing, a one-minute reset you can do anywhere.

Quick answers

What is HRV?
Heart rate variability: the variation in time between one heartbeat and the next.

Is a higher HRV always better?
No. Researchers describe an optimal level, what's normal varies from person to person, and some irregular heart rhythms can raise the number.

Why would HRV drop for a day?
A short night, an evening drink, a hard workout, stress or getting sick can all lower it. Watch the weekly trend instead of one reading.

How accurate are wearables for HRV?
It varies by device. Cleveland Clinic notes that an EKG or a chest-strap monitor is more reliable than a wrist sensor.

Can an HRV number diagnose a heart problem?
No. Talk with your doctor about any symptoms that concern you.

Enjoy the holiday

Questions? Call us at (949) 661-9777.

General information, not medical advice. Please talk with your own doctor about your situation.

Sources

1. Shaffer, F. & Ginsberg, J.P. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health, 5, 258.
2. Shaffer, F., McCraty, R. & Zerr, C.L. (2014). A healthy heart is not a metronome: an integrative review of the heart's anatomy and heart rate variability. Frontiers in Psychology, 5, 1040.
3. McCraty, R. & Shaffer, F. (2015). Heart rate variability: new perspectives on physiological mechanisms, assessment of self-regulatory capacity, and health risk. Global Advances in Health and Medicine, 4(1), 46-61.
4. Cleveland Clinic. Heart Rate Variability (HRV). https://my.clevelandclinic.org/health/symptoms/21773-heart-rate-variability-hrv
5. Stone, J.D. et al. (2021). Assessing the accuracy of popular commercial technologies that measure resting heart rate and heart rate variability. Frontiers in Sports and Active Living, 3, 585870.
6. Plews, D.J., Laursen, P.B., Stanley, J., Kilding, A.E. & Buchheit, M. (2013). Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Medicine, 43(9), 773-781.
7. Pietilä, J. et al. (2018). Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep in a large real-world sample of Finnish employees: observational study. JMIR Mental Health, 5(1), e23.
8. Natarajan, A., Su, H.-W. & Heneghan, C. (2020). Assessment of physiological signs associated with COVID-19 measured using wearable devices. npj Digital Medicine, 3, 156.