Sometimes, but not reliably. Your overnight HRV may dip and your resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate may rise a day or two before you feel ill, yet in studies this early signal caught only some infections and also went off on healthy days.
What happens to HRV when you get sick
When your body fights an infection, the calming branch of your nervous system steps back. Your heart beats a little faster at rest and the beat-to-beat variation shrinks. In data from 28,175 people who took a one-minute phone reading each morning, HRV ( RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD ) was about 10% lower on days they logged as sick, and resting heart rate was about 6% higher.
The question is whether these changes show up early enough, and clearly enough, to warn you. The illness page covers what happens while you are sick. This page is about the early warning.
What the studies found
Most of the evidence comes from COVID-19 research in 2020 and 2021.
- Fitbit users. Researchers used nightly breathing rate, resting heart rate and HRV from 1,257 people with symptoms. Their model was tuned to give few false alarms (95% specificity). With that setting, it flagged 15% of people the day before symptoms began and 24% on the day they began. Most cases were not flagged early.
- False alarms in healthy people. The same model, run on 300 healthy users, flagged about 4% of their days as sick. That is a little more than one false alarm a month.
- Smartwatch heart rate. Another study followed about 5,300 smartwatch wearers and found 32 with COVID-19. Changes in resting heart rate, steps or sleep showed up in 26 of them, often before symptoms. This study did not use HRV at all. Healthy people still got alerts about 0.66 times a month, often after travel, alcohol or stress.
- Apple Watch HRV. In 297 hospital workers, 13 tested positive. The daily rhythm of their HRV ( SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN ) changed in the 7 days before diagnosis. With so few cases, the authors called the results preliminary, and they did not report how well the watch could flag illness in one person.
So the signal is real at the group level, but it is weak for any one person. It works better when HRV is combined with resting heart rate and breathing rate. The study of 28,175 app users found HRV is sensitive but not specific: it moves with many stressors, not just infection. For spotting illness, its authors suggested resting heart rate may be the clearer marker. See HRV vs resting heart rate for how the two differ.
What to do with your own data
- Compare with your own baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline , not with other people. A single low night means little. Two or three low nights in a row, together with a higher resting heart rate, is more worth noticing.
- Rule out the obvious first. A late drink, a hard workout, a short night or a stressful day can each lower HRV without any infection.
- If your numbers are off and you feel a scratchy throat or unusual tiredness, take it easy. An easy day costs little if it turns out to be a false alarm.
- If your numbers are off and you feel fine, keep going as normal and check again tomorrow.
The lesson on building your HRV baseline shows how to tell a real change from day-to-day noise. If you are not sure whether a reading is low for you, see what a low HRV means.
When to talk to a doctor
HRV cannot tell you which illness you have or how serious it is. Go by your symptoms. Contact a doctor if you have a high fever that does not come down, trouble breathing, chest pain, confusion, or symptoms that keep getting worse. If you need to know whether you have an infection, such as flu or COVID-19, a proper test is the way to find out.