Illness and Recovery: Why Scores Change When You Are Unwell
Why resting heart rate, sleep and recovery scores shift when you are unwell, what smartwatch and Fitbit studies found about illness detection, and why a score is not a diagnosis or a clearance to train.
Content last updated . This page has not been medically reviewed.
Why scores change when you are sick
If a recovery or readiness score drops during a cold or flu, the device is probably picking up a real change in your body, not a malfunction. Researchers describe it this way: acute infections can raise resting heart rate and change daily routines because of the body's response to inflammation. [1] Wearables record these signals (resting heart rate, sleep time, activity), and company formulas fold them into a score. This page is education only. It cannot tell you whether you are ill, whether it is safe to train, or how to treat an illness.Evidence labels used here
Promising but limited means some positive research that is narrow, retrospective or unreplicated for consumer use. Insufficient evidence means too little research to judge. Marketing claim means a statement with no supporting study found.What research shows
| Question | What was found | Label |
|---|---|---|
| Can wearable data track illness across a population? | In 47,249 consistent Fitbit users in five US states, adding elevated resting heart rate and increased sleep to models improved influenza-like illness predictions in every state (average rise in correlation of 0.12). [1] | Promising but limited |
| Can a smartwatch flag one person's infection early? | In a study of 32 people with COVID-19 from a cohort of nearly 5,300, 26 (81%) showed changes in heart rate, steps or sleep; of 25 with symptom information, 22 were flagged before or at symptom onset. [2] | Promising but limited |
| Could a real-time alert work? | A retrospective analysis suggested 63% of cases could have been detected before symptoms using extreme resting-heart-rate elevations compared with a person's own baseline. [2] | Promising but limited |
| Does a consumer "you may be getting sick" score diagnose illness? | Proper evidence was not found in the sources we read for specific commercial scores. | Marketing claim |
Two cautions apply. First, these studies looked at particular devices and datasets (Fitbit and a smartwatch research cohort), so results do not transfer to every brand or model. Second, a change in resting heart rate is not specific to infection. The abstracts we read do not report how often changes occurred in people who were not ill, so they cannot tell you how often an alert would be a false alarm.
What a score cannot do while you are unwell
- It cannot tell what is wrong. A raised resting heart rate or lowered HRV is a signal, not a diagnosis. See resting heart rate and HRV explained.
- It cannot clear you to exercise.
- It cannot replace medical advice for symptoms. The NHS lists difficulty breathing, coughing up blood, leg pain or swelling and heart palpitations among reasons to contact NHS 111 (the UK urgent service). [3] In the US, for severe symptoms such as chest pain, call 911.
Illness and training in athletes
A systematic review of 54 studies (31,065 athletes and military recruits) found that acute respiratory illness symptoms lasted 7.1 days on average, and that 20.4% of illnesses led to more than one day lost from training or competition. [4] That describes what happened in studies. It is not a guide to when to return to exercise, and the authors noted current data are limited. [4] Questions about returning to exercise after an illness belong with your clinician, especially if you had a fever, chest symptoms or ongoing breathlessness.Practical, low-pressure habits
- Write down symptoms alongside the device data so you can tell your clinician how you actually felt.
- Do not push through illness or symptoms to protect a streak or restore a score.
- If numbers look unusual but you feel well, there is nothing here that says to act urgently. If you feel unwell with worrying symptoms, seek care regardless of the number.
Related guides
When scores conflict with how you feel, recovery scores explained, colds, flu and respiratory infections and fitness tracking: when professional assessment matters.Frequently asked questions
Why did my recovery score crash when I got a cold?
Infections can raise resting heart rate and change sleep and activity, and these are the signals that scores use. [1]
Can a smartwatch detect illness before symptoms?
In one research study of COVID-19, many cases showed changes before or at symptom onset. It was a research analysis, not a diagnostic test, and it does not show the same for other devices or illnesses. [2]
Should I train if my score is high but I feel sick?
A score cannot answer that. Do not use a number to override symptoms; ask a clinician if you are unsure.
When should I get medical help for symptoms during illness?
The NHS lists difficulty breathing, coughing up blood, leg pain or swelling and heart palpitations as reasons to contact NHS 111 in the UK. [3] In the US, call 911 for emergencies.
References
- Harnessing wearable device data to improve state-level real-time surveillance of influenza-like illness in the USA (Lancet Digit Health, 2020). Read 2026-10-22.
- Pre-symptomatic detection of COVID-19 from smartwatch data (Nat Biomed Eng, 2020). Read 2026-10-22.
- Shortness of breath, NHS (UK). Read 2026-10-22.
- Acute respiratory illness and return to sport: systematic review and meta-analysis (Br J Sports Med, 2022). Read 2026-10-22.