What is the red light under my Apple Watch?

Hey there henDerSon-70,

Thanks for reaching out to the Apple Support Communities. We'd be happy to provide you with the best information on this question.

To get started, are you referring to the red light on the back side of your Apple Watch that rests against your wrist? If so, this light is used to help measure your heart rate and blood oxygen (if using an Apple Watch Series 6 or Series 7).

For more details, check out the information listed in the following resources below:

• How to use the Blood Oxygen app on Apple Watch Series 6 or Series 7 - Apple Support
• Monitor your heart rate with Apple Watch - Apple Support

In the first resource you can find more information on the red light under the "About background measurements" section. In the second resource you can find more details on the light under the "How Apple Watch measures your heart rate" section.

We hope that's able to help.

Have a great day!