Tip: Apple Watch does not measure a blood oxygen at night – what you can do

I wear my Apple Watch overnight to capture the vital signs. The next morning, however, there are often no data for blood oxygen saturation. Why?

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A setting probably prevents the nightly measurement of the blood oxygen saturation. Since the red light used by the watch sensor slightly flashes between watch back and wrist, this automatically deactivates the theater mode of Watchos.

This mode thus prevents the recording. All other values ​​measured with green LEDs, including the heart rate, does not affect this.

Incidentally, the theater mode can be switched on easily, for example when you activate the “sleep” focus on the control center on the clock and the button with the two theater masks is in the immediate vicinity.

Check what is defined in “Settings> Blood oxygen”: Scroll down to check whether the measurements are allowed in the background. There you can switch or switch off for “sleeping in the focus” as well as for “in theater mode”. For safety, you can activate both settings, then the measurement should always work. In the watch app on the iPhone you will find the same settings in the “My Watch” tab under “Bluts oxygen”.

The blood oxygen app is available on the Apple Watch from Series 6 and on all ultra generations, but not on Watch SE. If a supported model does not measure the blood oxygen during the day, a hardware defect could be available – or you have acquired the clock on a trip to the USA within the past year.

On the models sold there, Apple has had to function since January 18, 2024 because of a Deactivate patent dispute. If the model number of the Apple Watch is marked with “LW/A” at the end, then you have such a device. You can find the number in “Settings> General> Info”.