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26 Sep 2024

5 ways a smartwatch can measure your vital health stats

Bec Summer

STACK Writer

Smartwatches and wearable fitness trackers provide peace of mind when it comes to basic health monitoring, but how do some of these new devices actually measure your vitals using clever tech?

Note: While health and fitness devices provide accurate readings, they are not intended to replace professional advice from a GP.

Heart rate

Measuring your heart rate is a basic but important function of most smartwatches and fitness trackers. Green LEDs on the underside of the watch lightly penetrate the skin of the wrist and this light is absorbed by the flow of blood.

As the blood vessels expand and contract, the watch measures changes in the concentration of red blood cells and an algorithm then converts changes in the light's intensity into a pulse rate.

Get ready for a great word: the process of measuring heart rate using light is called photoplethysmography.

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Blood oxygen (Sp02)

The level of oxygen saturation in your blood (SpO2) is a good indicator of how well your lungs are working, particularly during strenuous exercise, and is generally measured using pulse oximetry – that clip the doctor puts on your finger. Blood oxygen levels are generally 95%–100%, and a reading below 90% can be cause for concern.

SpO2 tracking is common feature of many smartwatches and works in a similar way to heart rate tracking, but in this case a red light wavelength is used to target haemoglobin (the protein in red blood cells that carries oxygen), which is then reflected back to the sensor to assess the differences between the light it's emitting and the reflected light.

Sleep patterns

A good night's sleep is vital for maintaining good health and wearing a smartwatch that supports sleep tracking to bed can give you a good insight into the quality of your kip and its various phases, from light and deep sleep, to REM (Rapid Eye Movement). Other measurements can include duration, how often you wake during the night, and the amount of light in your bedroom.

An accelerometer or gyroscope in the watch detects wrist motion and measures the amount of movement while you're sleeping – a technique known as actigraphy.

The watch's heart rate sensor also comes into play, because your heart rate slows during sleep and different sleep stages can create variables.

Once the data is collected, sleep trackers can assign you a sleep score based on movement, duration, and variations in your heart rate.

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Stress levels

We all suffer from stress and its negative impact on health is well known. Selected smartwatches can calculate your stress levels by measuring heart rate Variability (HVR) – changes in the time between each beat. Your HVR is controlled by the autonomic nervous system (ANS), which adjusts heart rate, blood pressure and breathing fluctuations according to physical stress.

A watch's optical heart rate sensor is generally used to assess stress levels, while more advanced models can have a dedicated electrodermal activity sensor that measures variations in your sweat levels.

An algorithm is used to assign a stress score – increased HVR indicates a low level of stress, while a lower reading means stress and fatigue is elevated.

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ECG (Electrocardiogram)

An electrocardiogram measures the health of your heart's rhythm and detects any abnormalities like atrial fibrillation, which causes an irregular heart rate that can lead to stroke.

While an ECG is generally performed by a doctor, it's also a feature available on selected smartwatches, which use a dedicated electrode sensor in the crown to measure the electrical activity of your heartbeat.

To activate the ECG sensor, the user simply presses a fingertip to the crown or watch face and an algorithm will assess heart rhythm, creating a readout graph in the companion app that can be sent to your GP for assessment.

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