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This would be an absolute game changer if they can pull this off. Blood pressure would be another great sensor to have.

With the new time in Daylight feature I could see Apple eventually introducing a UV sensor too - especially since the Health app already has a place to log the UV index.
It might be difficult for the sensor to distinguish between indirect daylight from indoor light. You'd have to wear the sensor on your head for it to work accurately. ;) Probably better to go with the weather station UV index and log it throughout the day.
 
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