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Smartwatches flash green to monitor heart rate. They use PhotoPlethysmoGraphy (PPG), which means that they flash a green light then measure how much light the blood absorbs at a given time, which helps them calculate a heart rate. Blood has a red color, so it reflects red light and absorbs green light.
Keep reading to learn more about how PPG works, whether there are any harmful effects from this light and how to stop the green light from flashing.
1. How Does the Green Light Monitor a Heart Rate?
Smartwatches have a green LED along with a photoelectric sensor that emits green light into the wearer’s blood vessel. This light and sensor combination helps determine the amount of blood flowing through the wrist over 24 hours. It uses the change in light intensity to calculate the heart rate.
Why Use a Green Light?
Green lights are usually for showing something to function correctly, so it can be a bit confusing to have a green LED that aims to measure someone’s heart rate instead. However, this is because that red in someone’s blood is about to absorb a large amount of green light.
Red light or any other color would reflect off the red blood, which wouldn’t give as accurate measurement for the smartwatch to calculate the heart rate, so it uses the green light along with the sensor.
2. Is the Green Light Harmful?
The green LED in a smartwatch belongs to the visible light, so there is no difference between it and natural green light. There is no harm to human skin, and since the sensor uses illumination and not some other method for sensing a heart rate, there is nothing to worry about from a health perspective about using a smartwatch to measure heart rate.
3. How Do I Stop the Green Light Blinking?
The only way to stop the green light from flashing is to turn off the automatic heart rate monitor, which, depending on which brand the smartwatch is, can turn off through the accompanying app or on the watch itself.
Keep in mind that turning off the automatic heart rate monitor means that there could be a limit to the information the smartwatch collects. It might help to turn it off only during the night while sleeping, so the flashing light doesn’t disturb anyone, then turn it back on in the morning.
How to Turm Off Automatic Heart Rate for Apple Watch
Open up the Watch App on the iPhone and tap “My Watch,” located at the bottom of the screen. In the following menu, there should be a “Privacy” option to click on. When the “Heart Rate” option appears, flip the switch to “off.”
How to Turn Off Automatic Heart Rate for Fitbit Watch
On the Fitbit, go to “Your Watch” settings on the top right of the screen. There should be a “Heart Rate” option and set it to “off” instead of “auto.”
Why is the Green Light Still Flashing?
If the green light is still flashing occasionally, it could be because the smartwatch brand uses the light and sensor for multiple features like REM and stress tracking. The settings for turning these automatic features off should be on menus similar to the heart rate monitor.
How to Turn Off Automatic REM Tracking
One of the most common smartwatches that automatically tracks REM and uses green LED and the sensor is the Samsun Galaxy Watch.
To turn off this setting:
- Scroll to the sleep tracking widget on the smartwatch and tap the three dots button on the right.
- In the new menu, tap the Settings option. There should be an option for REM tracking in the Settings, which on by default.
- Disable the monitoring, and the light should stay off.
Other brands of smartwatches should have a similar process to turn this feature off, though they may use the app instead of the smartwatch itself.
How to Turn Off Automatic Stress Tracking
There are a few different ways to get to the stress tracking settings depending on the brand of smartwatch, but generally, there should be a setting with an option close to “Health Monitoring.” Under these settings, there should be an “Automatic stress test/tracking” option. Disable this setting, and the light should stay off.
The settings may be on the smartwatch itself or the accompanying app depending on the brand.
4. How Do I Manually Check My Heart Rate?
If the automatic heart rate monitor is off, the wearer will need to manually check their heart rate to include it in their comprehensive health data.
Manual Heart Rate Check on the Apple Watch
Press the ‘Digital Crown,” then tap the “Heart Rate app,” which looks like a heart on a red background. This setting is usually where the most recent heart reading is, but there may not be one if the automatic heart rate monitor is off. Wait up to 10 minutes fr the smartwatch to display the current heart rate.
Manual Heart Rate Check on the Samsung Galaxy Watch
Press the power button to open the apps panel and tap the “Samsung Health” app. Scroll down until the heart rate section appears and tap it. Press the “Measure” option, and the smartwatch will start measuring the heart rate. There will be a circle progress bar to indicate how far the process is.
A report will appear on the smartwatch displaying the current heart rate and comparing it to the previous measure. Tap the “Tag” option to make a note.
5. Final Thoughts
The green LED light on a smartwatch will occasionally flash as it takes measurements to calculate heart rate. The red blood absorbs the green light, and how much is absorbed will tell the sensor on the smartwatch how much blood is circulating, resulting in a heart rate.
If the flashing light is bothersome, there is usually a way of disabling the automatic monitor by going into the settings on the accompanying app or on the smartwatch itself. Some smartwatches use the light and sensor for multiple purposes, so there may be more than one setting to switch off.
If you want to find the correct smartwatch for your need I recommend you to read the following article: The Correct SmartWatch for You in 2022! (Cheat Sheet!).
Espen is the Director of ProPairing and has written extensively about Bluetooth devices for years. He is a consumer product expert and has personally tested Bluetooth devices for the last decade.