Strapping an Apple Watch to your wrist is more than just a fashion statement, though there are a slew of different stylish bands out there made out of everything from sensible leather to funky beads. These devices also provide hands-free access to phone notifications, encourage wearers to stay active, and track levels of activity during work-outs. If you opt to add cellular support to your Apple Watch, you can also answer a call on the device, even when your phone isn’t nearby. Additionally, there are lots of things the Apple Watch can do without a phone at all, like stream music, track your sleep, and set alarms. It’s a little device that does a lot, but have you ever noticed an odd red glow coming from the watch now and then?
If you have an Apple Watch from Series 6 or later, you’ve likely spotted a red or green glow coming from the bottom of the device. A green glow means your smartwatch is checking your heart rate, which is a popular feature. A red glow indicates that the Apple gadget is checking your blood oxygen saturation levels. This indicates the amount of oxygen you have circulating in your blood and is an indicator of how well your heart and lungs are working.
Apple Watches use something called photoplethysmography, or PPG, sensors to monitor both heart rate and blood oxygen. The back of the watch has a light emitter and detector apertures that shine red, green, and near-infrared light into your skin and then measure the light that is reflected back. Because the protein in blood cells, called hemoglobin, absorbs light at different rates depending on how much oxygen it’s carrying, this technique can estimate your saturation levels.
Although this technology is not as accurate as a medical-grade device, much like the heart rate monitor, it can still detect changes that are worth looking into. For some individuals, this is just a neat trick but not particularly useful or necessary. However, if an Apple Watch user has certain health conditions, like chronic obstructive pulmonary disease or heart failure, it can be a convenient and important monitoring tool. It could also potentially alert you of an undiagnosed medical issue like sleep apnea.
To get the best results, Apple recommends that you wear the watch tightly, with the back crystal centered on your wrist and in contact with the soft tissue. Some people might have wrist bones that don’t allow the sensor to lay flat on the skin, in which case Apple recommends moving the device up a few centimeters on the arm. The company also notes that certain factors can affect the reliability of this tool, including tattoos, unusual motion, and a high heart rate.
Apple briefly removed the feature in 2024 after losing a patent case but was able to reintroduce it not long after. These days, most newer Apple Watches offer this health monitoring tool, though you may have to update your device and activate it in order to use it. To enable monitoring and view your blood oxygen data, you’ll have to use the Health app on your iPhone. If you don’t receive a prompt to set it up automatically when you open the Health app, follow these steps:
Tap the Browse tab
Tap Respiratory > Blood Oxygen > Enable
Open the Blood Oxygen app on your Apple Watch and measure your blood oxygen levels.
However, if you find the red light distracting, you can also disable this feature. On your phone, you will need to:
Open the Watch app
Select Blood Oxygen
Tap Disable Blood Oxygen Measurements
You can also disable this feature on the watch itself by pressing the digital crown, selecting the Settings app, and then going to Blood Oxygen.