These days, it's pretty normal for me to only sleep about 6 to 6 1/2 hours every night because I'm up working till one in the morning — and sometimes later — which made me realize that it's no wonder that I often feel like taking a nap by the time mid-afternoon rolls around.

I also sleep less now than I did when I was young, but for me it seems biological - not behavioral. Something flipped when I was in my 30s. I used to sleep deeply, 9 or 10 hours and more, and nothing could wake me up when I was sleeping. Sometime in my mid-30s, that changed to sleeping 7 hours a night and waking at every light sound. I rarely even bother with an alarm any more, because I wake up unaided before all but the earliest of time commitments.

On your main point, I am reminded of Ben Franklin's Diary.

First, he also didn't sleep much (I assume that was written when he was older ;-).

Second, for those blocks of time from 5-8am and 6-10pm, those hours would almost certainly be occupied by TV & social media for many/most people in the current generations.

Three or four hours of TV and doom-scrolling per person per day can kill a whole lot of productivity in a society. Although, to be fair, he had is own daily entertainment & social activities.