I imagine people went to bed earlier because light wasn't as abundant -
Yep. I think this is a big part of it. Also, supposedly camping for a week or two with only firelight and going to bed relatively early will have a huge effect on insomniacs. Something about resetting the sleep rhythm.
I was talking in more recent times, where the "external" entertainment like TV didn't run all night ad there wasn't much on past 9pm. As the availability of entertainment has extended longer, sleep has degraded accordingly I reckon.
wake up a midnight, read, have something to eat, sexy times,
Can you let my wife know that this is a thing?
I'm not sure that mindless brainwork is as much an issue as a sedentary lifestyle. If we don't use up energy moving our body then we'll use it up racing our brains.
I have tested this in myself, so n=1, but I think that while I can burn a lot of physical energy, it doesn't mean I sleep, if my brain hasn't done much thinking. However, a strenuous thought day, and I am incredibly tired. I wonder if there is a mechanism in the body to support the brain to recover when it is overworked, forcing sleep?
This is why anxiety can be relieved to a degree with exercise.
Yep - everyone should exercise, same as everyone should write/ have proper discussions/ meditate etc. I think it isn't a 1:1 relief though, and perhaps is part of a network of variables that improve overall condition. Most of us are pretty out of rhythm and unaligned for our best, as we favour some activities over another, in the same way that we favour sweet over healthy food.