"In the Fall of 2023, Apple introduced a new metric called “Time in Daylight” as part of watchOS 10 with Apple Watch Series SE (second generation) and Apple Watch Series 6 or later. This metric uses the ambient light sensor in Apple Watch and an associated algorithm to estimate how much time is spent in daylight and maximum intensity (lux) in 5-minute increments."

https://appleheartandmovementstudy.bwh.harvard.edu/summer-da...

Couldn't find any direct source from Apple tho