The circadian clock integrates photons over time, meaning consistent exposure to ambient light, even on overcast days, contributes to its signaling.

“the circadian clock the supermatic nucleus it sums photons it's a photon summing system so basically if you're outside in 8,000 Lux very overcast UK winter day and you're walking around hopefully without sunglasses because sunglasses are going to filter a lot of those photons out your circadian clock is summing the photons so it's an integration mechanism it's not triggered in a moment”

“on a cloudy day when you're outside it can be as bright as or an average of a 100,000 Lux but it won't seem that bright because you don't quote unquote see the Sun but it's also because when there's cloud cover a lot of those long wavelength of light such as orange and red light aren't coming through however and this is so important the circadian clock the supermatic nucleus it sums photons it's a photon summing system so basically if you're outside in 8,000 Lux very overcast UK winter day and you're walking around hopefully without sunglasses because sunglasses are going to filter a lot of those photons out your circadian clock is summing the photons so it's an integration mechanism it's not triggered in a moment”