Modern smartwatches are packed with health sensors. A single watch may track your heart rate around the clock, estimate blood oxygen, record an ECG, score your sleep and log changes in skin temperature overnight. These features can be useful, but they are easy to over-trust.
This explainer covers how each sensor works, what manufacturers and regulators say it can and cannot do, and why readings can go wrong. One point runs through the whole article: most smartwatch health features are wellness tools, not medical devices. Only specific features that have received regulatory clearance are intended for any kind of health screening, and none of them replaces a doctor. If you have a health concern, consult a qualified doctor.
Quick answer
- Heart rate: measured optically by shining light into the skin. Generally reasonable at rest, less reliable during intense exercise.
- SpO2 (blood oxygen): an optical estimate. On many watches it is explicitly labelled as not for medical use.
- ECG: a single-lead recording taken by touching the watch. Some ECG apps have regulatory clearance in certain countries, but they check for specific rhythms only and cannot detect a heart attack.
- Sleep: watches are fairly good at telling sleep from wake, but sleep stage breakdowns are estimates.
- Skin temperature: tracks overnight changes from your own baseline. It is not a thermometer and cannot check for fever on demand.
- Golden rule: use watch data to notice trends and start conversations with your doctor, never to diagnose yourself or delay care.
Wellness feature or medical device?
Regulators separate general wellness features from medical devices. In the United States, the FDA published a revised general wellness guidance in January 2026. According to a legal analysis of that guidance, non-invasive wearables that estimate values such as blood pressure, SpO2 or heart rate variability can be treated as wellness products, provided they are not intended to diagnose or treat a disease, do not prompt specific clinical action, and do not substitute for an authorised medical device.
Separately, some individual features go through regulatory review. Apple’s ECG app, sleep apnea notifications and hypertension notifications have received FDA authorisation in the US. In India, medical devices are overseen by the Central Drugs Standard Control Organisation (CDSCO), and Samsung has said its blood pressure and ECG features on Galaxy Watch received CDSCO clearance before launching here.
Note that clearance applies to a specific feature, not the whole watch, and clearance in one country does not mean the feature is available or approved in India.
Heart rate: optical sensors (PPG)
How it works
Most watches measure heart rate with photoplethysmography (PPG). Apple’s heart rate support page explains the principle: blood absorbs green light, so green LEDs paired with light-sensitive photodiodes detect the changing amount of blood flowing through the wrist with each heartbeat. Apple says its sensor can also use infrared light, for example during sleep, and supports readings from 30 to 210 beats per minute.
Accuracy caveats
Optical heart rate is generally most reliable at rest. A study published in the Journal of the American College of Cardiology in 2024 compared six PPG devices against ECG during treadmill stress tests in 81 people. The average error was 4.6 beats per minute at rest in normal rhythm, but rose to 13.8 at peak exercise, and was larger again in people with atrial fibrillation. The study tested older models, so newer devices may differ.
- A loose strap lets light leak in and is a common cause of odd readings.
- Movement, especially irregular wrist motion, can confuse the sensor.
- Cold weather and low blood flow to the skin can weaken the signal.
- Some tattoos can block the light the sensor relies on.
SpO2: blood oxygen estimates
How it works
Pulse oximetry uses light to estimate how much of your blood is carrying oxygen. The FDA describes pulse oximeters as devices that use light beams to estimate oxygen saturation and pulse rate without drawing blood.
What makers say it can and cannot do
Apple’s Blood Oxygen support page states that its measurements “are not intended for medical use and are only designed for general fitness and wellness purposes”. It lists skin perfusion (how much blood flows through the skin), cold, tattoos, motion and a loose fit as factors that affect readings, and says the feature is not available to people under 18.
Even medical-grade pulse oximeters have limits. The FDA lists poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use and fingernail polish among the factors that can affect accuracy, and advises people not to rely only on a pulse oximeter. If you feel breathless or unwell, seek medical advice regardless of what a watch shows.
ECG: electrical heart sensor
How it works
Unlike the optical sensor, an ECG feature measures the heart’s electrical activity. Apple says that on supported models, electrodes in the Digital Crown and the back of the watch form a closed circuit between your heart and both arms when you touch the crown. The result is similar to a single-lead ECG, whereas a hospital ECG typically uses multiple leads.
What it can and cannot do
Apple’s ECG app, which has been available in India since 2019, classifies a 30-second recording as sinus rhythm, atrial fibrillation (AFib) or inconclusive. Apple’s instructions for use are explicit about the limits:
- “The ECG app cannot check for signs of a heart attack.”
- It is not intended for people under 22 years old.
- It is not recommended for people with other known arrhythmias.
- Users should not take clinical action based on the output without consulting a qualified healthcare professional.
Apple’s 2019 announcement also notes the app does not detect blood clots, stroke, heart failure or other heart conditions. If you have symptoms such as chest pain or pressure, fainting or severe breathlessness, treat it as an emergency and seek medical help immediately rather than taking a watch reading.
Sleep tracking
How it works
Watches estimate sleep mainly from wrist movement and heart rate patterns, sometimes combined with other signals.
Accuracy caveats
The gold standard for measuring sleep is polysomnography in a sleep lab. A Sleep Foundation summary of recent research reports that consumer trackers show promise at telling sleep from wake, but are less precise at classifying sleep stages, and that some may overestimate total sleep by missing short periods of wakefulness. Treat stage percentages as rough estimates.
Sleep apnea notifications
Some watches now offer cleared screening features. Apple’s sleep apnoea notifications use the accelerometer to look for “breathing disturbances” and may alert you if these are consistently elevated over a 30-day period. Apple states the feature is for people 18 and older who have not been diagnosed, is “not intended to diagnose, treat or aid in the management of sleep apnoea”, and that not all people with sleep apnoea will receive a notification. Availability varies by country.
Skin and wrist temperature
How it works
Temperature sensing on watches tracks small changes rather than a single reading. Apple’s wrist temperature page says supported models sample temperature every five seconds while you sleep and establish a baseline after about five nights. The watch then shows how each night compares with that baseline.
What it can and cannot do
Apple states plainly that the feature “is not a medical device and is not intended for use in medical diagnosis”, and that it “is not a thermometer and cannot provide wrist temperature measurements on demand”. Readings can shift with diet, exercise, alcohol, sleeping environment, menstrual cycles, illness and fit. If you think you have a fever, use a proper thermometer.
How to get more useful readings
- Wear the watch snugly but comfortably. Makers such as Apple stress that a good fit matters.
- Keep the sensor area clean and dry.
- Stay still for spot checks such as SpO2 and ECG.
- Compare trends over weeks, not single readings.
- Check which features are officially available in India.
- Never stop, start or change medication based on watch data without medical advice.
Frequently asked questions
Is my smartwatch a medical device?
Usually not. Most features are designed for general wellness. A few specific features, such as certain ECG apps, have received regulatory clearance in some countries, but that applies to the feature, not the whole watch. Check the maker’s documentation for your country.
Can a smartwatch detect a heart attack?
No. Apple’s own instructions state that its ECG app cannot check for signs of a heart attack. If you have warning symptoms such as chest pain, seek emergency medical help immediately.
Why does my watch show low SpO2 sometimes?
Wrist-based readings can be affected by a loose fit, movement, cold, low skin blood flow and tattoos. If you feel unwell or breathless, consult a doctor.
Can I use wrist temperature to check for fever?
No. Apple says its wrist temperature feature is not a thermometer and cannot give on-demand readings. Use a proper thermometer and consult a doctor if you are concerned.
Sources
- Ropes and Gray: FDA updated guidances on general wellness products (January 2026)
- Samsung Newsroom India: Samsung Health Monitor BP and ECG India debut
- Apple: Monitor your heart rate with Apple Watch
- American College of Cardiology: Accuracy of wearable heart rate monitors
- US FDA: Pulse oximeters
- Apple: How to use the Blood Oxygen app
- Apple Newsroom India: ECG app available in India
- Apple: ECG app instructions for use
- Sleep Foundation: Accuracy of sleep trackers
- Apple: Sleep apnoea notifications
- Apple: Track nightly wrist temperature changes
- Cardiovascular Business: FDA clears Apple Watch hypertension algorithm
- TechCrunch: Apple Watch sleep apnea feature and FDA
