I imagine people went to bed earlier because light wasn't as abundant - it cost money or effort to fuel lanterns or fires - so it was just easier to go to bed when it was dark. I read that for a long time in human history, people would sleep twice a night... maybe they'd go to bed at 7pm... wake up a midnight, read, have something to eat, sexy times, etc go back to sleep at 2am and wake up at 6am... the one sleep a night apparently is a recent invention with the light blub.
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. This is why anxiety can be relieved to a degree with exercise.
Super interesting about your brain wiring being so different now. The brain is endlessly fascinating.