but probably everyone has noticed they’re more lethargic on cloudy days.

Not everyone; I can focus and think a lot more easily without bright light, and of course everyone who closes their eyes when in deep thought is doing that because they want the darkness. I suspect this could be related to how some people like to work early hours, while others prefer later hours.

"some people like to work early hours, while others prefer later hours."

And some like me like to switch. Either early morning or late at night. Basically when most other people are asleep and not bothering the flow.

But few things make me as awake and clear minded, as a bright sunny day coming after rainy days. Then I cannot be inside a cave.