Illness and Recovery: Why Scores Change When You Are Unwell

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

1. Can wearable data track illness across a population?

   Promising but limited

   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]

2. Can a smartwatch flag one person's infection early?

   Promising but limited

   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]

3. Could a real-time alert work?

   Promising but limited

   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]

4. Does a consumer "you may be getting sick" score diagnose illness?

   Marketing claim

   Proper evidence was not found in the sources we read for specific commercial scores.

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.