The Invisible Sensor
Modern Apple Watches (Series 6 and newer) feature a specialized optical sensor on the back of the device designed to measure your Blood Oxygen (SpO2) levels. By default, the watch does not wait for you to open the app; it actively fires red and infrared LEDs into your wrist periodically in the background throughout the day and night to log the data to Apple Health. While this provides a comprehensive medical overview, firing those bright LEDs frequently drains a significant amount of battery. If you only want to measure your blood oxygen manually when you feel sick, you must shut down the background scanner.
How to Turn Off Background Blood Oxygen Measurements
You can disable the automated background polling via the watch settings.
- Open the Settings app directly on your Apple Watch (the grey gear icon).
- Scroll down the main list and tap on Blood Oxygen.
- Ensure the main “Blood Oxygen Measurements” switch remains On (so you can still use the app manually).
- Look below it for the “Allow Background Measurements” section.
- You will see two toggle switches: In Sleep Focus and In Theater Mode.
- Toggle both of these switches to the Off position.
Manual Scans Only
The change is absolute. The Apple Watch will immediately stop firing the red LEDs into your wrist while you sleep or work. The sensor will remain completely dormant, saving a noticeable amount of battery life. The only time the watch will ever measure your SpO2 levels is if you actively launch the Blood Oxygen app and explicitly press the “Start” button.