Most smartwatches estimate your heart rate, blood oxygen (SpO2), sleep and steps with a handful of small sensors on the back and inside the case. Knowing how each one works helps you read the numbers sensibly and understand their limits.
Please note: smartwatch health readings are estimates for general wellness and fitness, not medical use. They cannot diagnose, treat or rule out any condition. If you feel unwell, or a reading worries you, consult a doctor.
The main sensors at a glance
| Sensor | What it tracks | How it works |
|---|---|---|
| Optical heart-rate sensor | Heart rate; also feeds sleep, stress and calorie estimates | LEDs shine light into the skin and a light detector picks up tiny changes as blood pulses |
| Red and infrared light sensor | Blood oxygen (SpO2) estimate | Compares how red and infrared light are absorbed by the blood |
| Accelerometer | Steps, movement, sleep and raise-to-wake | Measures the movement and direction of your wrist |
| Gyroscope | Rotation, used for workouts and gestures | Measures how your wrist twists and turns |
| GPS (some models) | Route, distance and pace outdoors | Uses satellite signals to work out your position |
Spec sheets describe these in different ways. The Redmi Watch 5 Lite, for example, lists a heart-rate sensor with a blood-oxygen sensor, an accelerometer, a gyroscope and in-built GPS.
Heart rate
The optical sensor (often called a PPG sensor) measures heart rate continuously or at set intervals, which is what "24/7 heart rate" on a spec sheet means. It works best when the watch sits snugly and your arm is fairly still.
Readings can drift when:
- the strap is loose, so outside light leaks in;
- you do exercises that flex the wrist, such as weights or racquet sports;
- your skin is cold and blood flow to the wrist drops;
- there are tattoos under the sensor, or sweat and dirt on it.
Look at trends, such as your resting heart rate over a few weeks, rather than a single number.
Blood oxygen (SpO2)
SpO2 readings on most watches are spot checks. For the most consistent result, sit down, rest your arm on a table, keep the watch snug just above the wrist bone and stay still until the reading finishes. Wrist readings vary more than fingertip readings because of movement, fit and circulation, so don't rely on them to monitor a health condition.
Sleep
Sleep tracking combines movement from the accelerometer with heart-rate patterns to estimate when you fell asleep, when you woke up and which stage (light, deep or REM) you were probably in. Stage labels are estimates. The most useful information is usually the simple part: how long you slept and how regular your bedtime is.
Steps, distance and calories
- Steps come from the accelerometer recognising the rhythm of your arm swing. Counts can rise on a bumpy auto-rickshaw ride and fall when your hands are still, for example while pushing a trolley.
- Distance is estimated from your steps and stride length unless the watch has GPS. The Noise Pulse 4 Max lists distance among its metrics, and its listing notes that it has no GPS.
- Calories are estimated from your profile (age, height and weight), heart rate and movement, so keep your profile up to date in the app.
Stress and other wellness features
Stress scores are usually worked out from heart-rate variability, the small changes in time between heartbeats. The Fastrack Limitless Glide, for instance, lists an auto stress monitor along with 24x7 heart-rate monitoring, SpO2 and sleep monitoring. Some watches also list blood pressure features; a wrist estimate without a cuff is not a substitute for a validated blood pressure monitor. Cycle tracking mostly works from the dates you log in the app.
How to get more consistent readings
- Wear the watch snugly, about a finger's width above your wrist bone, and a little tighter during workouts.
- Keep the sensor window on the back of the watch clean and dry.
- Keep your profile (age, height and weight) up to date in the companion app.
- Take spot readings, such as SpO2, the same way each time: seated, still and with your arm resting.
- Install firmware updates from the app, as updates can include tracking improvements.
Quick answers
Can a smartwatch replace a medical device?
No. Smartwatch readings are for general wellness, not medical use. If you need to diagnose or monitor a condition, use the devices and tests your doctor recommends.
Why does my SpO2 reading change every time I check it?
Small movements, a loose strap, cold hands and the position of your arm all affect optical readings. Take readings the same way each time, sitting still with your arm resting.
Do I need GPS for step counting?
No. Steps come from the motion sensor. GPS helps measure the route and distance of outdoor walks, runs and rides more precisely.
Compare models in our smartwatch collection, or read our Smartwatch Buying Guide for India before you choose.
