Heart rate variability, or HRV, is how much the time between your heartbeats changes from one beat to the next, measured in milliseconds. It often drops when you’re getting sick because fighting an infection pushes your body toward its stress response: your heart beats faster and more evenly, so there’s less variation between beats.
Here’s what HRV measures, what else moves it, and how to read your own numbers.
What HRV actually measures
Your heart doesn’t beat like a metronome. At 60 beats per minute, the gaps between beats aren’t all exactly one second. One might be 0.95 seconds and the next 1.08. Cleveland Clinic describes HRV as these small changes, a fraction of a second added or taken away between beats.
That variation comes mostly from your autonomic nervous system, the part that runs without you thinking about it. It has two sides:
- The sympathetic side speeds things up. It’s the “fight or flight” response to exercise, stress, danger or illness.
- The parasympathetic side slows things down. It works mainly through the vagus nerve and takes over when you’re resting, digesting and sleeping.
When the resting side is in charge, the time between beats shifts more freely, for example speeding up a little as you breathe in and slowing as you breathe out. So HRV tends to be higher when you’re rested and lower when your body is under strain. A review in Frontiers in Public Health covers how this works and the different ways HRV is calculated.
Why HRV drops when you’re sick
An infection is a stress your body has to deal with, and the response looks a lot like other kinds of stress. Your immune system ramps up, you may run a fever, and your heart rate goes up. In one study of young men with acute infections, heart rate rose by about 8.5 beats per minute for each degree Celsius of fever, and it stayed high even during sleep.
A faster heart has less room between beats to vary, and the stress response pulls back the resting side that creates much of that variation. The result is lower HRV.
Wearable studies show this in everyday life:
- In a Fitbit study of people with COVID-19, breathing rate and heart rate typically went up with illness while HRV went down.
- In the Warrior Watch study, health care workers wore Apple Watches, and their HRV patterns changed in the week before a positive COVID-19 test as well as after it.
Fitbit’s own guidance in the Google Health app puts it simply: a significant drop in HRV can be a sign of stress, strain or illness.
A drop in HRV doesn’t tell you what’s wrong, only that your body is under more load. It tends to come back up as you recover, which can take days.
Other things that lower or raise HRV
Illness is one cause among many. These can lower your HRV for a night or a few days:
- Alcohol. In a study of more than 4,000 Finnish workers, drinking lowered nighttime recovery measured by HRV, and more drinks meant a bigger drop. Even a small amount showed an effect.
- Hard exercise. After a single hard workout, the resting side of your nervous system can take 24 to 48 hours or more to fully recover. Easy sessions recover faster.
- Poor or short sleep, and stress. Cleveland Clinic lists both, along with anxiety and depression.
- The menstrual cycle. A 2026 review of wearable studies found HRV tends to be higher early in the cycle and 3 to 9 percent lower toward the end.
- Age. HRV generally goes down as you get older.
Regular exercise, good sleep and lower stress tend to raise it over weeks and months.
How to read your own HRV
Compare yourself to yourself. HRV differs a lot between people because of age, sex, fitness and genetics. A number that’s low for one person can be high for another, so comparing with a friend tells you very little. Compare tonight with your own last two weeks.
Look at the trend, not one night. A single low night after a late dinner and two glasses of wine isn’t news. A drop that holds for several days, especially with a higher resting heart rate, is more worth noticing.
Read it with your other numbers. When you’re getting sick, HRV often drops at the same time as resting heart rate and breathing rate rise. If all three move together and you feel off, that tells you more than any one of them.
Know which HRV you’re looking at. Devices don’t all measure it the same way:
- Apple Watch reports SDNN, the spread of the gaps between beats, taken at moments through the day and night.
- Fitbit and the Google Health app report RMSSD, which looks at the change from one beat to the next, mostly during sleep.
The two give different numbers for the same heart. If you switch devices, start your comparison over rather than lining up the old and new numbers.
Treat the exact number loosely. A wrist sensor estimates HRV from your pulse, and Cleveland Clinic notes that an EKG is more accurate. Wearables are most useful for spotting changes in your own trend over time.
This isn’t medical advice (see our health disclaimer). If you feel unwell, or a change in your numbers comes with symptoms that worry you, talk to a doctor.
How Voke shows HRV
Voke reads HRV from Apple Health or Google Health and keeps SDNN and RMSSD apart. The History tab shows your HRV over the last 7 days, 30 days or year, with your latest value and how far it is from your average over the earlier days. When your device records HRV through the night, the Sleep screen shows that night’s HRV as a curve with its average, as covered in reading your sleep chart. Voke shows what your device recorded. It doesn’t diagnose anything.
Sources
- Cleveland Clinic: Heart rate variability (HRV)
- Shaffer and Ginsberg, Frontiers in Public Health, 2017: An overview of heart rate variability metrics and norms
- Karjalainen and Viitasalo, Archives of Internal Medicine, 1986: Fever and cardiac rhythm
- Natarajan et al., npj Digital Medicine, 2020: Assessment of physiological signs associated with COVID-19 measured using wearable devices
- Hirten et al., Journal of Medical Internet Research, 2021: Use of physiological data from a wearable device to identify SARS-CoV-2 infection and symptoms
- Pietilä et al., JMIR Mental Health, 2018: Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep
- Stanley, Peake and Buchheit, Sports Medicine, 2013: Cardiac parasympathetic reactivation following exercise
- Sports Medicine, 2026: Wearable-derived heart rate variability across the menstrual cycle, hormonal contraceptive use, and reproductive life stages in females
- Google Health Help: Track your vitals in the Google Health app
- Apple Developer: heartRateVariabilitySDNN