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There is something I don't understand about the theory that exposure to bright light early in the morning keeps our circadian rhythm on schedule. Supposedly, our natural circadian rhythm would have a period of slightly more than 24 hours if we were kept in a dark room, but exposure to light in the morning keeps it on a 24 hour schedule. The problem with this theory is that, in a world where the sun were rising a little later, we would still be exposed to lots of sunlight early in the morning. It would just be delayed slightly. So how does waking up in the morning and exposing oneself to bright light tell the body what the sun's schedule is? Wouldn't our bodies have to know exactly when the sun was rising and then make the appropriate adjustments based on the time of year and weather? Isn't the only way to do this to be outside all morning for an entire year? How does the unreliable signal of early morning sun exposure contain enough information to make slight adjustments to our circadian rhythm?
I think it's pretty plausible that the body sees bright sunlight and says "Aha, the sun is up! Nice! Next time we go to bed, we should definitely wake up about 22 hours from now." And that would keep it well-aligned to be awake just before sunrise.
It doesn't have to know when the sun will rise, just when the sun did rise yesterday, and then it's close enough.
So, it needs to know almost exactly when the sun rose. That doesn't work with variable weather. If your brain is basing it on brightness, it's going to take a lot longer to reach a given brightness on an overcast day than on a clear day.
You also need to be outside where you can see that change in brightness. Following the common advice of getting some light exposure within the first few hours of waking is not going to be able to fine tune a circadian rhythm that is off by half an hour.
It's entirely plausible that your brain kinda groks light angle. And it's entirely possible that your brain kinda groks the overall change in brightness in a day, so if it gets bright suddenly, it thinks "ah, this is the real brightness, the previous one was overcast or whatever".
Also, again, remember that the change is slow. Even if it gets only one good anchor every week or so, maybe that's enough to keep it aligned with the sunrise, everything else just keeps it vaguely on-track.
The human brain is complicated enough that I'm willing to give it a lot of slack in terms of what it may be capable of.
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