Heart rate when sick: how wearables spot illness early

Yes, heart rate usually goes up when you are sick, mainly because of fever. Each 1 °C rise in body temperature raised heart rate by about 8.5 beats per minute (bpm) in a study of young men with acute infections, and their heart rate stayed high even during sleep. Wearable studies show resting heart rate can rise before symptoms start.

That early rise is why smartwatches and rings can hint at illness. In a Stanford study of 3,318 people, an alerting system based on heart rate and steps flagged 80% of COVID-19 infections, a median of 3 days before symptoms. The same signals also appear after alcohol, stress or travel, so a rise is a prompt to pay attention, not a diagnosis. For normal values, see the resting heart rate by age chart.

Why does heart rate go up when you are sick?

Heart rate goes up during infection mainly because of fever and the body's inflammatory response. A higher body temperature speeds up the heart, by about 8.5 bpm per 1 °C in one study. Infections can also change sleep and daily activity. Together these push resting heart rate above your usual range.

The 1986 study recorded 24-hour heart rate in 27 young men during an uncomplicated febrile infection (adenovirus, influenza, Mycoplasma and others) and again after recovery:

Measure	During fever	After recovery
Average axillary temperature	38.4 °C	Normal
Average heart rate (24 hours)	84.0 bpm	66.5 bpm
Change per 1 °C	About +8.5 bpm	-
Heart rate during sleep	Stayed high	Normal dip

Source: Karjalainen and Viitasalo, Arch Intern Med 1986. Small study of young men; values in women, older adults and people on heart medicines may differ.

Can a wearable detect illness before symptoms?

Studies show wearables can often detect a change before symptoms, but not always. In one Stanford study, 63% of COVID-19 cases could have been flagged before symptom onset in real time, based on unusual rises in resting heart rate against each person's baseline. A later prospective system flagged 80% of infections, a median of 3 days early.

Study	Device data	Main finding
Radin et al. 2020, 47,249 Fitbit users	Resting heart rate, sleep	Adding wearable data improved state-level flu-like illness predictions in all 5 states studied
Mishra et al. 2020, 32 COVID-19 cases from about 5,300 people	Heart rate, steps, sleep	26 of 32 (81%) had changes; 63% could have been detected before symptoms in real time
Alavi et al. 2022, 3,318 people, 84 infected	Heart rate, steps	Alerts in 67 of 84 (80%), median 3 days before symptoms; stress, alcohol and travel also triggered alerts
Hirten et al. 2022, 407 health care workers	Apple Watch HRV	Model found infection with sensitivity 82% and specificity 77%; HRV pattern features were top predictors

Sources: as listed. These are research systems, not consumer diagnostic tools.

The key word in all of these studies is baseline. The systems did not use a fixed threshold like 80 bpm. They compared each person with their own recent values. A rise from 55 to 62 bpm can be a strong signal for one person and noise for another.

What happens to HRV and temperature when you are sick?

During an infection, HRV (heart rate variability) tends to fall while resting heart rate rises, and wrist or finger temperature can rise with fever. In a study of 407 health care workers wearing Apple Watches, HRV pattern features were among the top predictors of infection. A ring study found that illness-linked rises in skin temperature matched self-reported fever.

Signal	Typical direction during infection	Best way to read it
Resting heart rate	Up	Against your own 7 to 30-day baseline
HRV	Down	Against your own baseline, same device and method
Wrist or skin temperature	Up with fever	Change from baseline, not absolute degrees
Respiratory rate	Can change	Against your own baseline
Sleep duration	Often longer	Week-over-week

Sources: Karjalainen and Viitasalo 1986; Hirten et al. 2022; Smarr et al. 2020; Radin et al. 2020 (sleep).

Apple's Vitals app uses the same idea. It sets a typical range for each overnight metric after about 7 nights and notifies you when multiple metrics are outside that range, listing illness, alcohol, medicines and elevation as possible factors. Apple says these measurements are not intended for medical use. For HRV ranges, see low HRV meaning. For temperature, see Apple Watch wrist temperature.

What else can look like being sick?

Alcohol, stress, travel and other everyday events can raise resting heart rate and lower HRV, which looks similar to the start of an illness. In the 2022 alerting study, stress, alcohol and travel triggered alerts too, at about 1.15 alert days per person compared with 3.42 for COVID-19.

Before deciding you are getting sick, check the last 2 or 3 days:

• Alcohol: Alcohol also triggered alerts in the 2022 study.
• Sleep: Compare last night's sleep with your usual length and bedtime.
• Travel and stress: Both triggered alerts in the 2022 study.
• Cycle: If you track your cycle, compare with the same phase in earlier months.
• Symptoms: Sore throat, runny nose, aches and low fever are the classic early cold signs.

If none of these explain the change and you feel run down, the signal deserves more weight. The full list of causes is in why is my resting heart rate going up.

Should you exercise with an elevated heart rate when sick?

When your resting heart rate is well above your baseline and you have a fever, rest is the safer choice. Fever already raises heart rate by about 8.5 bpm per 1 °C, so exercise starts from a higher level. Wait until the fever is gone and your resting heart rate is back near your usual range.

Ease back in with light activity once you feel better and your morning numbers are back near your usual range. If you have chest pain, palpitations or shortness of breath during or after an illness, stop and see a doctor before training again.

When should you see a doctor?

See a doctor if your resting heart rate stays above 100 bpm, if you have a high or persistent fever, or if you have shortness of breath, chest pain, palpitations, confusion, fainting or signs of dehydration. People with heart or lung conditions, pregnant people and older adults should call earlier. Get emergency care for severe symptoms.

A wearable can tell you that your body is doing something unusual. It can not tell you what the illness is, how serious it is, or whether you need treatment. Use the trend to decide when to rest, when to test and when to call, and share it with your doctor. Adults get about 2 to 3 colds a year on average, according to the CDC, so a few flagged weeks a year can be normal. If you seem to be sick much more often, read why am I getting sick so often.

How Svanu looks for early illness signals

Svanu includes illness detection. It compares your resting heart rate, HRV, wrist temperature, blood oxygen and respiratory rate with your own recent baseline, using prior days only so that a sick day does not raise its own reference. It flags a change only when it is both statistically unusual and large enough to matter. It is a wellness signal, never a diagnosis.

Resting heart rate comes from Apple Health, or is derived from your sleep heart-rate samples when your watch does not write it. Wrist temperature, blood oxygen and respiratory rate come from Apple Health when your watch provides them. All scores are computed on your iPhone, with no health database on a server. Read more about illness detection in Svanu.