@Jeruba My guess is it isn’t a natural or instinctual thing.
I think it has more to do with the work week. I don’t know the history, but my guess is people used to work 6 days a week, and rested on the 7th, partly from religious teachings. It’s possible religion came up with that because of some sort of agreement that came from natural instinct, or maybe the people who wrote that down wanted a rest every so often themselves, and after 6 days they were all exhausted, and agreed that they needed a rest after that. More likely they wanted people to come into the place of worship every “week” to donate their money. Interestingly, though, in Judaism you can’t touch money on the Sabbath. I guess Christians realized there was opportunity there.
Modern day we have a 5 day work week (usually) and then we fit other chores in where we can. 5 days straight is actually very hard for me. I prefer a more erratic schedule, even if I’m working the same amount of total days in the end.
When they created the calendar, I think that started to routinize how we group days and weeks together. The Jewish calendar creates a month different than our modern-day calendar, but I think people reference a month’s time according to the calendar being used, not some sort of natural instinct.
The moon gives us a 28 day cycle, but we mostly ignore that, because our current calendar ignores it.
I think if a new calendar came out with an eight day week, we would adjust to it accordingly.
My natural instinct is to not be on a rigid routine. I prefer doing things more randomly, but outside forces make me conform to some extent.
Or, maybe you’re right, and it is like circadian rhythm, I’m just putting some thoughts out there after considering your hypothesis.